diff --git a/.env.example b/.env.example
index d3e03c26..ed50bd57 100644
--- a/.env.example
+++ b/.env.example
@@ -37,7 +37,7 @@ MAIL_PORT=1025
 MAIL_USERNAME=null
 MAIL_PASSWORD=null
 MAIL_ENCRYPTION=null
-MAIL_FROM_ADDRESS="hello@example.com"
+MAIL_FROM_ADDRESS="no-reply@solidtime.test"
 MAIL_FROM_NAME="${APP_NAME}"
 
 AWS_ACCESS_KEY_ID=
diff --git a/.gitignore b/.gitignore
index e6028879..ffdee140 100644
--- a/.gitignore
+++ b/.gitignore
@@ -26,3 +26,4 @@ yarn-error.log
 /blob-report/
 /playwright/.cache/
 /coverage
+/extensions/*
diff --git a/README.md b/README.md
index eb0c4796..4f7a64f7 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,7 @@ Add the following entry to your `/etc/hosts`
 ```
 127.0.0.1 solidtime.test
 127.0.0.1 playwright.solidtime.test
+127.0.0.1 mail.solidtime.test
 ```
 
 ## Running E2E Tests
diff --git a/app/Actions/Fortify/CreateNewUser.php b/app/Actions/Fortify/CreateNewUser.php
index 292463ef..3702cf5a 100644
--- a/app/Actions/Fortify/CreateNewUser.php
+++ b/app/Actions/Fortify/CreateNewUser.php
@@ -6,9 +6,12 @@ namespace App\Actions\Fortify;
 
 use App\Models\Organization;
 use App\Models\User;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Hash;
 use Illuminate\Support\Facades\Validator;
+use Illuminate\Validation\ValidationException;
+use Korridor\LaravelModelValidationRules\Rules\UniqueEloquent;
 use Laravel\Fortify\Contracts\CreatesNewUsers;
 use Laravel\Jetstream\Jetstream;
 
@@ -20,12 +23,27 @@ class CreateNewUser implements CreatesNewUsers
      * Create a newly registered user.
      *
      * @param  array<string, string>  $input
+     *
+     * @throws ValidationException
      */
     public function create(array $input): User
     {
         Validator::make($input, [
-            'name' => ['required', 'string', 'max:255'],
-            'email' => ['required', 'string', 'email', 'max:255', 'unique:users'],
+            'name' => [
+                'required',
+                'string',
+                'max:255',
+            ],
+            'email' => [
+                'required',
+                'string',
+                'email',
+                'max:255',
+                new UniqueEloquent(User::class, 'email', function (Builder $builder): Builder {
+                    /** @var Builder<User> $builder */
+                    return $builder->where('is_placeholder', '=', false);
+                }),
+            ],
             'password' => $this->passwordRules(),
             'terms' => Jetstream::hasTermsAndPrivacyPolicyFeature() ? ['accepted', 'required'] : '',
         ])->validate();
diff --git a/app/Actions/Jetstream/AddOrganizationMember.php b/app/Actions/Jetstream/AddOrganizationMember.php
index 47f9ccfa..7a84d53f 100644
--- a/app/Actions/Jetstream/AddOrganizationMember.php
+++ b/app/Actions/Jetstream/AddOrganizationMember.php
@@ -8,8 +8,10 @@ use App\Models\Organization;
 use App\Models\User;
 use Closure;
 use Illuminate\Contracts\Validation\Rule;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Support\Facades\Gate;
 use Illuminate\Support\Facades\Validator;
+use Korridor\LaravelModelValidationRules\Rules\ExistsEloquent;
 use Laravel\Jetstream\Contracts\AddsTeamMembers;
 use Laravel\Jetstream\Events\AddingTeamMember;
 use Laravel\Jetstream\Events\TeamMemberAdded;
@@ -21,21 +23,24 @@ class AddOrganizationMember implements AddsTeamMembers
     /**
      * Add a new team member to the given team.
      */
-    public function add(User $user, Organization $organization, string $email, ?string $role = null): void
+    public function add(User $owner, Organization $organization, string $email, ?string $role = null): void
     {
-        Gate::forUser($user)->authorize('addTeamMember', $organization);
+        Gate::forUser($owner)->authorize('addTeamMember', $organization);
 
         $this->validate($organization, $email, $role);
 
-        $newTeamMember = Jetstream::findUserByEmailOrFail($email);
+        $newOrganizationMember = User::query()
+            ->where('email', $email)
+            ->where('is_placeholder', '=', false)
+            ->firstOrFail();
 
-        AddingTeamMember::dispatch($organization, $newTeamMember);
+        AddingTeamMember::dispatch($organization, $newOrganizationMember);
 
         $organization->users()->attach(
-            $newTeamMember, ['role' => $role]
+            $newOrganizationMember, ['role' => $role]
         );
 
-        TeamMemberAdded::dispatch($organization, $newTeamMember);
+        TeamMemberAdded::dispatch($organization, $newOrganizationMember);
     }
 
     /**
@@ -46,9 +51,7 @@ class AddOrganizationMember implements AddsTeamMembers
         Validator::make([
             'email' => $email,
             'role' => $role,
-        ], $this->rules(), [
-            'email.exists' => __('We were unable to find a registered user with this email address.'),
-        ])->after(
+        ], $this->rules())->after(
             $this->ensureUserIsNotAlreadyOnTeam($organization, $email)
         )->validateWithBag('addTeamMember');
     }
@@ -61,7 +64,13 @@ class AddOrganizationMember implements AddsTeamMembers
     protected function rules(): array
     {
         return array_filter([
-            'email' => ['required', 'email', 'exists:users'],
+            'email' => [
+                'required',
+                'email',
+                (new ExistsEloquent(User::class, 'email', function (Builder $builder) {
+                    return $builder->where('is_placeholder', '=', false);
+                }))->withMessage(__('We were unable to find a registered user with this email address.')),
+            ],
             'role' => Jetstream::hasRoles()
                             ? ['required', 'string', new Role]
                             : null,
@@ -75,7 +84,7 @@ class AddOrganizationMember implements AddsTeamMembers
     {
         return function ($validator) use ($team, $email) {
             $validator->errors()->addIf(
-                $team->hasUserWithEmail($email),
+                $team->hasRealUserWithEmail($email),
                 'email',
                 __('This user already belongs to the team.')
             );
diff --git a/app/Actions/Jetstream/InviteOrganizationMember.php b/app/Actions/Jetstream/InviteOrganizationMember.php
index a688fffa..686a9978 100644
--- a/app/Actions/Jetstream/InviteOrganizationMember.php
+++ b/app/Actions/Jetstream/InviteOrganizationMember.php
@@ -34,6 +34,7 @@ class InviteOrganizationMember implements InvitesTeamMembers
 
         InvitingTeamMember::dispatch($organization, $email, $role);
 
+        /** @var TeamInvitation $invitation */
         $invitation = $organization->teamInvitations()->create([
             'email' => $email,
             'role' => $role,
@@ -50,9 +51,7 @@ class InviteOrganizationMember implements InvitesTeamMembers
         Validator::make([
             'email' => $email,
             'role' => $role,
-        ], $this->rules($organization), [
-            'email.unique' => __('This user has already been invited to the team.'),
-        ])->after(
+        ], $this->rules($organization))->after(
             $this->ensureUserIsNotAlreadyOnTeam($organization, $email)
         )->validateWithBag('addTeamMember');
     }
@@ -68,10 +67,10 @@ class InviteOrganizationMember implements InvitesTeamMembers
             'email' => [
                 'required',
                 'email',
-                new UniqueEloquent(OrganizationInvitation::class, 'email', function (Builder $builder) use ($organization) {
+                (new UniqueEloquent(OrganizationInvitation::class, 'email', function (Builder $builder) use ($organization) {
                     /** @var Builder<OrganizationInvitation> $builder */
                     return $builder->whereBelongsTo($organization, 'organization');
-                }),
+                }))->withMessage(__('This user has already been invited to the team.')),
             ],
             'role' => Jetstream::hasRoles()
                             ? ['required', 'string', new Role]
@@ -86,7 +85,7 @@ class InviteOrganizationMember implements InvitesTeamMembers
     {
         return function ($validator) use ($organization, $email) {
             $validator->errors()->addIf(
-                $organization->hasUserWithEmail($email),
+                $organization->hasRealUserWithEmail($email),
                 'email',
                 __('This user already belongs to the team.')
             );
diff --git a/app/Exceptions/Api/ApiException.php b/app/Exceptions/Api/ApiException.php
new file mode 100644
index 00000000..bcd5ae01
--- /dev/null
+++ b/app/Exceptions/Api/ApiException.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Api;
+
+use Exception;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+use LogicException;
+
+abstract class ApiException extends Exception
+{
+    /**
+     * Render the exception into an HTTP response.
+     */
+    public function render(Request $request): JsonResponse
+    {
+        return response()
+            ->json([
+                'error' => true,
+                'key' => $this->getKey(),
+                'message' => $this->getTranslatedMessage(),
+            ], 400);
+    }
+
+    /**
+     * Get the key for the exception.
+     */
+    public function getKey(): string
+    {
+        if (defined(static::class.'::KEY')) {
+            return static::KEY;
+        }
+
+        throw new LogicException('API exceptions need the KEY constant defined.');
+    }
+
+    /**
+     * Get the translated message for the exception.
+     */
+    public function getTranslatedMessage(): string
+    {
+        return __('exceptions.api.'.$this->getKey());
+    }
+}
diff --git a/app/Exceptions/Api/TimeEntryStillRunningApiException.php b/app/Exceptions/Api/TimeEntryStillRunningApiException.php
new file mode 100644
index 00000000..e110a9ae
--- /dev/null
+++ b/app/Exceptions/Api/TimeEntryStillRunningApiException.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Api;
+
+class TimeEntryStillRunningApiException extends ApiException
+{
+    const string KEY = 'time_entry_still_running';
+}
diff --git a/app/Exceptions/Api/UserNotPlaceholderApiException.php b/app/Exceptions/Api/UserNotPlaceholderApiException.php
new file mode 100644
index 00000000..2aae88c9
--- /dev/null
+++ b/app/Exceptions/Api/UserNotPlaceholderApiException.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Api;
+
+class UserNotPlaceholderApiException extends ApiException
+{
+    const string KEY = 'user_not_placeholder';
+}
diff --git a/app/Exceptions/ApiException.php b/app/Exceptions/ApiException.php
deleted file mode 100644
index c4a61eab..00000000
--- a/app/Exceptions/ApiException.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace App\Exceptions;
-
-use Exception;
-use Illuminate\Http\JsonResponse;
-use Illuminate\Http\Request;
-
-class ApiException extends Exception
-{
-    /**
-     * Render the exception into an HTTP response.
-     */
-    public function render(Request $request): JsonResponse
-    {
-        return response()
-            ->json([
-                'error' => true,
-                'message' => $this->getMessage(),
-            ], 400);
-    }
-}
diff --git a/app/Exceptions/TimeEntryStillRunning.php b/app/Exceptions/TimeEntryStillRunning.php
deleted file mode 100644
index a4ee00a2..00000000
--- a/app/Exceptions/TimeEntryStillRunning.php
+++ /dev/null
@@ -1,9 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace App\Exceptions;
-
-class TimeEntryStillRunning extends ApiException
-{
-}
diff --git a/app/Http/Controllers/Api/V1/TimeEntryController.php b/app/Http/Controllers/Api/V1/TimeEntryController.php
index 0b627b1d..514da8e4 100644
--- a/app/Http/Controllers/Api/V1/TimeEntryController.php
+++ b/app/Http/Controllers/Api/V1/TimeEntryController.php
@@ -4,7 +4,7 @@ declare(strict_types=1);
 
 namespace App\Http\Controllers\Api\V1;
 
-use App\Exceptions\TimeEntryStillRunning;
+use App\Exceptions\Api\TimeEntryStillRunningApiException;
 use App\Http\Requests\V1\TimeEntry\TimeEntryIndexRequest;
 use App\Http\Requests\V1\TimeEntry\TimeEntryStoreRequest;
 use App\Http\Requests\V1\TimeEntry\TimeEntryUpdateRequest;
@@ -102,7 +102,7 @@ class TimeEntryController extends Controller
     /**
      * Create time entry
      *
-     * @throws AuthorizationException|TimeEntryStillRunning
+     * @throws AuthorizationException|TimeEntryStillRunningApiException
      */
     public function store(Organization $organization, TimeEntryStoreRequest $request): JsonResource
     {
@@ -114,8 +114,7 @@ class TimeEntryController extends Controller
 
         if ($request->get('end') === null && TimeEntry::query()->where('user_id', $request->get('user_id'))->where('end', null)->exists()) {
             // TODO: API documentation
-            // TODO: Create concept for api exceptions
-            throw new TimeEntryStillRunning('User already has an active time entry');
+            throw new TimeEntryStillRunningApiException();
         }
 
         $timeEntry = new TimeEntry();
diff --git a/app/Http/Controllers/Api/V1/UserController.php b/app/Http/Controllers/Api/V1/UserController.php
new file mode 100644
index 00000000..94ff6f2e
--- /dev/null
+++ b/app/Http/Controllers/Api/V1/UserController.php
@@ -0,0 +1,56 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api\V1;
+
+use App\Exceptions\Api\UserNotPlaceholderApiException;
+use App\Http\Requests\V1\User\UserIndexRequest;
+use App\Http\Resources\V1\User\UserCollection;
+use App\Models\Organization;
+use App\Models\User;
+use Illuminate\Auth\Access\AuthorizationException;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+use Laravel\Jetstream\Contracts\InvitesTeamMembers;
+
+class UserController extends Controller
+{
+    /**
+     * List all users in an organization
+     *
+     * @throws AuthorizationException
+     */
+    public function index(Organization $organization, UserIndexRequest $request): UserCollection
+    {
+        $this->checkPermission($organization, 'users:view');
+
+        $users = $organization->users()
+            ->paginate();
+
+        return UserCollection::make($users);
+    }
+
+    /**
+     * Invite a placeholder user to become a real user in the organization
+     *
+     * @throws AuthorizationException|UserNotPlaceholderApiException
+     */
+    public function invitePlaceholder(Organization $organization, User $user, Request $request): JsonResponse
+    {
+        $this->checkPermission($organization, 'users:invite-placeholder');
+
+        if (! $user->is_placeholder) {
+            throw new UserNotPlaceholderApiException();
+        }
+
+        app(InvitesTeamMembers::class)->invite(
+            $request->user(),
+            $organization,
+            $user->email,
+            'employee'
+        );
+
+        return response()->json($user);
+    }
+}
diff --git a/app/Http/Requests/V1/User/UserIndexRequest.php b/app/Http/Requests/V1/User/UserIndexRequest.php
new file mode 100644
index 00000000..f600d01b
--- /dev/null
+++ b/app/Http/Requests/V1/User/UserIndexRequest.php
@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\V1\User;
+
+use App\Models\Organization;
+use Illuminate\Contracts\Validation\ValidationRule;
+use Illuminate\Foundation\Http\FormRequest;
+
+/**
+ * @property Organization $organization
+ */
+class UserIndexRequest extends FormRequest
+{
+    /**
+     * Get the validation rules that apply to the request.
+     *
+     * @return array<string, array<string|ValidationRule>>
+     */
+    public function rules(): array
+    {
+        return [
+        ];
+    }
+}
diff --git a/app/Http/Resources/V1/User/UserCollection.php b/app/Http/Resources/V1/User/UserCollection.php
new file mode 100644
index 00000000..e9461a8f
--- /dev/null
+++ b/app/Http/Resources/V1/User/UserCollection.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Resources\V1\User;
+
+use Illuminate\Http\Resources\Json\ResourceCollection;
+
+class UserCollection extends ResourceCollection
+{
+    /**
+     * The resource that this resource collects.
+     *
+     * @var string
+     */
+    public $collects = UserResource::class;
+}
diff --git a/app/Http/Resources/V1/User/UserResource.php b/app/Http/Resources/V1/User/UserResource.php
new file mode 100644
index 00000000..acb3b9d4
--- /dev/null
+++ b/app/Http/Resources/V1/User/UserResource.php
@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Resources\V1\User;
+
+use App\Http\Resources\V1\BaseResource;
+use App\Models\Membership;
+use App\Models\User;
+use Illuminate\Http\Request;
+
+/**
+ * @property User $resource
+ */
+class UserResource extends BaseResource
+{
+    /**
+     * Transform the resource into an array.
+     *
+     * @return array<string, string|bool|int|null|array<string>>
+     */
+    public function toArray(Request $request): array
+    {
+        /** @var Membership $membership */
+        $membership = $this->resource->getRelationValue('membership');
+
+        return [
+            /** @var string $id ID */
+            'id' => $this->resource->id,
+            /** @var string $name Name */
+            'name' => $this->resource->name,
+            /** @var string $email Email */
+            'email' => $this->resource->email,
+            /** @var string $role Role */
+            'role' => $membership->role,
+            /** @var bool $is_placeholder Placeholder user for imports, user might not really exist and does not know about this placeholder membership */
+            'is_placeholder' => $this->resource->is_placeholder,
+        ];
+    }
+}
diff --git a/app/Listeners/RemovePlaceholder.php b/app/Listeners/RemovePlaceholder.php
new file mode 100644
index 00000000..4ba70a9d
--- /dev/null
+++ b/app/Listeners/RemovePlaceholder.php
@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Listeners;
+
+use App\Models\User;
+use App\Service\UserService;
+use Laravel\Jetstream\Events\TeamMemberAdded;
+
+class RemovePlaceholder
+{
+    /**
+     * Handle the event.
+     */
+    public function handle(TeamMemberAdded $event): void
+    {
+        /** @var UserService $userService */
+        $userService = app(UserService::class);
+        $placeholders = User::query()
+            ->where('is_placeholder', '=', true)
+            ->where('email', '=', $event->user->email)
+            ->belongsToOrganization($event->team)
+            ->get();
+
+        foreach ($placeholders as $placeholder) {
+            $userService->assignOrganizationEntitiesToDifferentUser($event->team, $placeholder, $event->user);
+        }
+    }
+}
diff --git a/app/Models/Organization.php b/app/Models/Organization.php
index 62bd33b7..284675d4 100644
--- a/app/Models/Organization.php
+++ b/app/Models/Organization.php
@@ -8,6 +8,7 @@ use Database\Factories\OrganizationFactory;
 use Illuminate\Database\Eloquent\Collection;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 use Laravel\Jetstream\Events\TeamCreated;
 use Laravel\Jetstream\Events\TeamDeleted;
@@ -59,4 +60,30 @@ class Organization extends JetstreamTeam
         'updated' => TeamUpdated::class,
         'deleted' => TeamDeleted::class,
     ];
+
+    /**
+     * Get all the non-placeholder users of the organization including its owner.
+     *
+     * @return Collection<User>
+     */
+    public function allRealUsers(): Collection
+    {
+        return $this->realUsers->merge([$this->owner]);
+    }
+
+    public function hasRealUserWithEmail(string $email): bool
+    {
+        return $this->allRealUsers()->contains(function (User $user) use ($email): bool {
+            return $user->email === $email;
+        });
+    }
+
+    /**
+     * @return BelongsToMany<User>
+     */
+    public function realUsers(): BelongsToMany
+    {
+        return $this->users()
+            ->where('is_placeholder', false);
+    }
 }
diff --git a/app/Models/User.php b/app/Models/User.php
index 19e6bc8f..a58a737f 100644
--- a/app/Models/User.php
+++ b/app/Models/User.php
@@ -6,6 +6,8 @@ namespace App\Models;
 
 use Database\Factories\UserFactory;
 use Filament\Panel;
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Collection;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Relations\BelongsToMany;
@@ -21,9 +23,16 @@ use Laravel\Passport\HasApiTokens;
  * @property string $id
  * @property string $name
  * @property string $email
+ * @property string|null $email_verified_at
+ * @property string|null $password
+ * @property bool $is_placeholder
+ * @property Collection<Organization> $organizations
+ * @property Collection<TimeEntry> $timeEntries
  *
  * @method HasMany<Organization> ownedTeams()
  * @method static UserFactory factory()
+ * @method static Builder<User> query()
+ * @method Builder<User> belongsToOrganization(Organization $organization)
  */
 class User extends Authenticatable
 {
@@ -97,4 +106,27 @@ class User extends Authenticatable
             ->withTimestamps()
             ->as('membership');
     }
+
+    /**
+     * @return HasMany<TimeEntry>
+     */
+    public function timeEntries(): HasMany
+    {
+        return $this->hasMany(TimeEntry::class);
+    }
+
+    /**
+     * @param  Builder<User>  $builder
+     * @return Builder<User>
+     */
+    public function scopeBelongsToOrganization(Builder $builder, Organization $organization): Builder
+    {
+        return $builder->where(function (Builder $builder) use ($organization): Builder {
+            return $builder->whereHas('organizations', function (Builder $query) use ($organization): void {
+                $query->whereKey($organization->getKey());
+            })->orWhereHas('ownedTeams', function (Builder $query) use ($organization): void {
+                $query->whereKey($organization->getKey());
+            });
+        });
+    }
 }
diff --git a/app/Providers/EventServiceProvider.php b/app/Providers/EventServiceProvider.php
index ee09f108..4dc848b5 100644
--- a/app/Providers/EventServiceProvider.php
+++ b/app/Providers/EventServiceProvider.php
@@ -4,10 +4,11 @@ declare(strict_types=1);
 
 namespace App\Providers;
 
+use App\Listeners\RemovePlaceholder;
 use Illuminate\Auth\Events\Registered;
 use Illuminate\Auth\Listeners\SendEmailVerificationNotification;
 use Illuminate\Foundation\Support\Providers\EventServiceProvider as ServiceProvider;
-use Illuminate\Support\Facades\Event;
+use Laravel\Jetstream\Events\TeamMemberAdded;
 
 class EventServiceProvider extends ServiceProvider
 {
@@ -20,6 +21,9 @@ class EventServiceProvider extends ServiceProvider
         Registered::class => [
             SendEmailVerificationNotification::class,
         ],
+        TeamMemberAdded::class => [
+            RemovePlaceholder::class,
+        ],
     ];
 
     /**
diff --git a/app/Providers/JetstreamServiceProvider.php b/app/Providers/JetstreamServiceProvider.php
index 1de94d23..e3757c7c 100644
--- a/app/Providers/JetstreamServiceProvider.php
+++ b/app/Providers/JetstreamServiceProvider.php
@@ -75,6 +75,8 @@ class JetstreamServiceProvider extends ServiceProvider
             'organizations:view',
             'organizations:update',
             'import',
+            'users:invite-placeholder',
+            'users:view',
         ])->description('Administrator users can perform any action.');
 
         Jetstream::role('manager', 'Manager', [
@@ -95,6 +97,7 @@ class JetstreamServiceProvider extends ServiceProvider
             'tags:update',
             'tags:delete',
             'organizations:view',
+            'users:view',
         ])->description('Editor users have the ability to read, create, and update.');
 
         Jetstream::role('employee', 'Employee', [
diff --git a/app/Service/Import/Importers/ImporterProvider.php b/app/Service/Import/Importers/ImporterProvider.php
index 4ed1033f..ed413566 100644
--- a/app/Service/Import/Importers/ImporterProvider.php
+++ b/app/Service/Import/Importers/ImporterProvider.php
@@ -6,6 +6,9 @@ namespace App\Service\Import\Importers;
 
 class ImporterProvider
 {
+    /**
+     * @var array<string, class-string<ImporterContract>>
+     */
     private array $importers = [
         'toggl_time_entries' => TogglTimeEntriesImporter::class,
     ];
diff --git a/app/Service/UserService.php b/app/Service/UserService.php
new file mode 100644
index 00000000..e554e078
--- /dev/null
+++ b/app/Service/UserService.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use App\Models\Organization;
+use App\Models\TimeEntry;
+use App\Models\User;
+
+class UserService
+{
+    public function assignOrganizationEntitiesToDifferentUser(Organization $organization, User $fromUser, User $toUser): void
+    {
+        // Time entries
+        dump(TimeEntry::query()
+            ->whereBelongsTo($organization, 'organization')
+            ->whereBelongsTo($fromUser, 'user')
+            ->update([
+                'user_id' => $toUser->getKey(),
+            ]));
+    }
+}
diff --git a/config/telescope.php b/config/telescope.php
index 21d8ddda..d4057545 100644
--- a/config/telescope.php
+++ b/config/telescope.php
@@ -155,7 +155,7 @@ return [
 
         Watchers\LogWatcher::class => [
             'enabled' => env('TELESCOPE_LOG_WATCHER', true),
-            'level' => 'error',
+            'level' => 'debug',
         ],
 
         Watchers\MailWatcher::class => env('TELESCOPE_MAIL_WATCHER', true),
diff --git a/database/factories/OrganizationFactory.php b/database/factories/OrganizationFactory.php
index e0d629e4..e1efdea6 100644
--- a/database/factories/OrganizationFactory.php
+++ b/database/factories/OrganizationFactory.php
@@ -27,10 +27,10 @@ class OrganizationFactory extends Factory
         ];
     }
 
-    public function withOwner(): self
+    public function withOwner(?User $owner = null): self
     {
         return $this->state(fn (array $attributes) => [
-            'user_id' => User::factory(),
+            'user_id' => $owner === null ? User::factory() : $owner,
         ]);
     }
 }
diff --git a/database/factories/UserFactory.php b/database/factories/UserFactory.php
index 60f31ffb..893c7e65 100644
--- a/database/factories/UserFactory.php
+++ b/database/factories/UserFactory.php
@@ -31,9 +31,19 @@ class UserFactory extends Factory
             'remember_token' => Str::random(10),
             'profile_photo_path' => null,
             'current_team_id' => null,
+            'is_placeholder' => false,
         ];
     }
 
+    public function placeholder(bool $placeholder = true): static
+    {
+        return $this->state(function (array $attributes) use ($placeholder): array {
+            return [
+                'is_placeholder' => $placeholder,
+            ];
+        });
+    }
+
     /**
      * Indicate that the model's email address should be unverified.
      */
diff --git a/database/seeders/DatabaseSeeder.php b/database/seeders/DatabaseSeeder.php
index 7dd88fa8..b59a486d 100644
--- a/database/seeders/DatabaseSeeder.php
+++ b/database/seeders/DatabaseSeeder.php
@@ -22,31 +22,57 @@ class DatabaseSeeder extends Seeder
     public function run(): void
     {
         $this->deleteAll();
-        $organization1 = Organization::factory()->create([
+        $userAcmeOwner = User::factory()->create([
+            'name' => 'ACME Admin',
+            'email' => 'owner@acme.test',
+        ]);
+        $organizationAcme = Organization::factory()->withOwner($userAcmeOwner)->create([
             'name' => 'ACME Corp',
         ]);
-        $user1 = User::factory()->withPersonalOrganization()->create([
+        $userAcmeManager = User::factory()->withPersonalOrganization()->create([
             'name' => 'Test User',
             'email' => 'test@example.com',
         ]);
-        $employee1 = User::factory()->withPersonalOrganization()->create([
-            'name' => 'Test User',
-            'email' => 'employee@example.com',
-        ]);
-        $userAcmeAdmin = User::factory()->create([
+        $userAcmeAdmin = User::factory()->withPersonalOrganization()->create([
             'name' => 'ACME Admin',
             'email' => 'admin@acme.test',
         ]);
-        $user1->organizations()->attach($organization1, [
+        $userAcmeEmployee = User::factory()->withPersonalOrganization()->create([
+            'name' => 'Max Mustermann',
+            'email' => 'max.mustermann@acme.test',
+        ]);
+        $userAcmePlaceholder = User::factory()->placeholder()->create([
+            'name' => 'Old Employee',
+            'email' => 'old.employee@acme.test',
+            'password' => null,
+        ]);
+        $userAcmeManager->organizations()->attach($organizationAcme, [
             'role' => 'manager',
         ]);
-        $userAcmeAdmin->organizations()->attach($organization1, [
+        $userAcmeAdmin->organizations()->attach($organizationAcme, [
             'role' => 'admin',
         ]);
-        $timeEntriesEmployees = TimeEntry::factory()
+        $userAcmeEmployee->organizations()->attach($organizationAcme, [
+            'role' => 'employee',
+        ]);
+        $userAcmePlaceholder->organizations()->attach($organizationAcme, [
+            'role' => 'employee',
+        ]);
+
+        $timeEntriesAcmeAdmin = TimeEntry::factory()
             ->count(10)
-            ->forUser($employee1)
-            ->forOrganization($organization1)
+            ->forUser($userAcmeAdmin)
+            ->forOrganization($organizationAcme)
+            ->create();
+        $timeEntriesAcmePlaceholder = TimeEntry::factory()
+            ->count(10)
+            ->forUser($userAcmePlaceholder)
+            ->forOrganization($organizationAcme)
+            ->create();
+        $timeEntriesAcmePlaceholder = TimeEntry::factory()
+            ->count(10)
+            ->forUser($userAcmeEmployee)
+            ->forOrganization($organizationAcme)
             ->create();
         $client = Client::factory()->create([
             'name' => 'Big Company',
@@ -63,11 +89,11 @@ class DatabaseSeeder extends Seeder
         $organization2 = Organization::factory()->create([
             'name' => 'Rival Corp',
         ]);
-        $user1 = User::factory()->withPersonalOrganization()->create([
+        $userAcmeManager = User::factory()->withPersonalOrganization()->create([
             'name' => 'Other User',
             'email' => 'test@rival-company.test',
         ]);
-        $user1->organizations()->attach($organization2, [
+        $userAcmeManager->organizations()->attach($organization2, [
             'role' => 'admin',
         ]);
         $otherCompanyProject = Project::factory()->forClient($client)->create([
diff --git a/docker-compose.yml b/docker-compose.yml
index 876bcfe0..d4faaf13 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -57,10 +57,43 @@ services:
                 - '${DB_USERNAME}'
             retries: 3
             timeout: 5s
-    mailpit:
-        image: 'axllent/mailpit:latest'
+    pgsql_test:
+        image: 'postgres:15'
+        environment:
+            PGPASSWORD: '${DB_PASSWORD:-secret}'
+            POSTGRES_DB: '${DB_DATABASE}'
+            POSTGRES_USER: '${DB_USERNAME}'
+            POSTGRES_PASSWORD: '${DB_PASSWORD:-secret}'
+        volumes:
+            - 'sail-pgsql-test:/var/lib/postgresql/data'
+            - './vendor/laravel/sail/database/pgsql/create-testing-database.sql:/docker-entrypoint-initdb.d/10-create-testing-database.sql'
         networks:
             - sail
+        healthcheck:
+            test:
+                - CMD
+                - pg_isready
+                - '-q'
+                - '-d'
+                - '${DB_DATABASE}'
+                - '-U'
+                - '${DB_USERNAME}'
+            retries: 3
+            timeout: 5s
+    mailpit:
+        image: 'axllent/mailpit:latest'
+        labels:
+            - "traefik.enable=true"
+            - "traefik.docker.network=${NETWORK_NAME}"
+            - "traefik.http.routers.solidtime-mailpit.rule=Host(`mail.${NGINX_HOST_NAME}`)"
+            - "traefik.http.routers.solidtime-mailpit.entrypoints=web"
+            - "traefik.http.services.solidtime-mailpit.loadbalancer.server.port=8025"
+            - "traefik.http.routers.solidtime-mailpit-https.rule=Host(`mail.${NGINX_HOST_NAME}`)"
+            - "traefik.http.routers.solidtime-mailpit-https.entrypoints=websecure"
+            - "traefik.http.routers.solidtime-mailpit-https.tls=true"
+        networks:
+            - sail
+            - reverse-proxy
     playwright:
         image: mcr.microsoft.com/playwright:v1.41.1-jammy
         command: ['npx', 'playwright', 'test', '--ui-port=8080', '--ui-host=0.0.0.0']
@@ -88,3 +121,5 @@ networks:
 volumes:
     sail-pgsql:
         driver: local
+    sail-pgsql-test:
+        driver: local
diff --git a/lang/en/exceptions.php b/lang/en/exceptions.php
new file mode 100644
index 00000000..706574fb
--- /dev/null
+++ b/lang/en/exceptions.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+use App\Exceptions\Api\TimeEntryStillRunningApiException;
+use App\Exceptions\Api\UserNotPlaceholderApiException;
+
+return [
+    'api' => [
+        TimeEntryStillRunningApiException::KEY => 'Time entry is still running',
+        UserNotPlaceholderApiException::KEY => 'The given user is not a placeholder',
+    ],
+];
diff --git a/phpunit.xml b/phpunit.xml
index 82235621..f5ab132a 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -22,6 +22,7 @@
         <env name="BCRYPT_ROUNDS" value="4"/>
         <env name="CACHE_DRIVER" value="array"/>
         <env name="DB_CONNECTION" value="pgsql"/>
+        <env name="DB_HOST" value="pgsql_test"/>
         <env name="MAIL_MAILER" value="array"/>
         <env name="PULSE_ENABLED" value="false"/>
         <env name="QUEUE_CONNECTION" value="sync"/>
diff --git a/routes/api.php b/routes/api.php
index a75683a9..ca46d271 100644
--- a/routes/api.php
+++ b/routes/api.php
@@ -8,6 +8,7 @@ use App\Http\Controllers\Api\V1\OrganizationController;
 use App\Http\Controllers\Api\V1\ProjectController;
 use App\Http\Controllers\Api\V1\TagController;
 use App\Http\Controllers\Api\V1\TimeEntryController;
+use App\Http\Controllers\Api\V1\UserController;
 use Illuminate\Support\Facades\Route;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
@@ -29,6 +30,12 @@ Route::middleware('auth:api')->prefix('v1')->name('v1.')->group(static function
         Route::put('/organizations/{organization}', [OrganizationController::class, 'update'])->name('update');
     });
 
+    // User routes
+    Route::name('users.')->group(static function () {
+        Route::get('/organizations/{organization}/users', [UserController::class, 'index'])->name('index');
+        Route::post('/organizations/{organization}/users/{user}/invite-placeholder', [UserController::class, 'invitePlaceholder'])->name('invite-placeholder');
+    });
+
     // Project routes
     Route::name('projects.')->group(static function () {
         Route::get('/organizations/{organization}/projects', [ProjectController::class, 'index'])->name('index');
diff --git a/tests/Feature/InviteTeamMemberTest.php b/tests/Feature/InviteTeamMemberTest.php
index a4f14b29..60bffb6b 100644
--- a/tests/Feature/InviteTeamMemberTest.php
+++ b/tests/Feature/InviteTeamMemberTest.php
@@ -4,9 +4,11 @@ declare(strict_types=1);
 
 namespace Tests\Feature;
 
+use App\Models\TimeEntry;
 use App\Models\User;
 use Illuminate\Foundation\Testing\RefreshDatabase;
 use Illuminate\Support\Facades\Mail;
+use Illuminate\Support\Facades\URL;
 use Laravel\Jetstream\Mail\TeamInvitation;
 use Tests\TestCase;
 
@@ -31,6 +33,49 @@ class InviteTeamMemberTest extends TestCase
         $this->assertCount(1, $user->currentTeam->fresh()->teamInvitations);
     }
 
+    public function test_team_member_can_not_be_invited_to_team_if_already_on_team(): void
+    {
+        // Arrange
+        Mail::fake();
+        $user = User::factory()->withPersonalOrganization()->create();
+        $existingUser = User::factory()->create();
+        $user->currentTeam->users()->attach($existingUser, ['role' => 'admin']);
+        $this->actingAs($user);
+
+        // Act
+        $response = $this->post('/teams/'.$user->currentTeam->id.'/members', [
+            'email' => $existingUser->email,
+            'role' => 'admin',
+        ]);
+
+        // Assert
+        $response->assertInvalid(['email'], 'addTeamMember');
+        Mail::assertNotSent(TeamInvitation::class);
+        $this->assertCount(0, $user->currentTeam->fresh()->teamInvitations);
+    }
+
+    public function test_team_member_can_be_invited_to_team_if_already_on_team_as_placeholder(): void
+    {
+        // Arrange
+        Mail::fake();
+        $user = User::factory()->withPersonalOrganization()->create();
+        $existingUser = User::factory()->create([
+            'is_placeholder' => true,
+        ]);
+        $user->currentTeam->users()->attach($existingUser, ['role' => 'employee']);
+        $this->actingAs($user);
+
+        // Act
+        $response = $this->post('/teams/'.$user->currentTeam->id.'/members', [
+            'email' => $existingUser->email,
+            'role' => 'employee',
+        ]);
+
+        // Assert
+        Mail::assertSent(TeamInvitation::class);
+        $this->assertCount(1, $user->currentTeam->fresh()->teamInvitations);
+    }
+
     public function test_team_member_invitations_can_be_cancelled(): void
     {
         // Arrange
@@ -49,4 +94,97 @@ class InviteTeamMemberTest extends TestCase
         // Assert
         $this->assertCount(0, $user->currentTeam->fresh()->teamInvitations);
     }
+
+    public function test_team_member_invitations_can_be_accepted(): void
+    {
+        // Arrange
+        Mail::fake();
+        $owner = User::factory()->withPersonalOrganization()->create();
+        $user = User::factory()->withPersonalOrganization()->create();
+        $invitation = $owner->currentTeam->teamInvitations()->create([
+            'email' => $user->email,
+            'role' => 'employee',
+        ]);
+        $this->actingAs($user);
+
+        // Act
+        $acceptUrl = URL::temporarySignedRoute(
+            'team-invitations.accept',
+            now()->addMinutes(60),
+            [$invitation->getKey()]
+        );
+        $response = $this->get($acceptUrl);
+
+        // Assert
+        $this->assertCount(0, $owner->currentTeam->fresh()->teamInvitations);
+        $user->refresh();
+        $this->assertCount(1, $user->organizations);
+        $this->assertContains($owner->currentTeam->getKey(), $user->organizations->pluck('id'));
+    }
+
+    public function test_team_member_invitations_of_placeholder_can_be_accepted_and_migrates_date_to_real_user(): void
+    {
+        // Arrange
+        Mail::fake();
+        $placeholder = User::factory()->withPersonalOrganization()->create([
+            'is_placeholder' => true,
+        ]);
+
+        $owner = User::factory()->withPersonalOrganization()->create();
+        $owner->currentTeam->users()->attach($placeholder, ['role' => 'employee']);
+        $timeEntries = TimeEntry::factory()->forOrganization($owner->currentTeam)->forUser($placeholder)->createMany(5);
+
+        $user = User::factory()->withPersonalOrganization()->create([
+            'email' => $placeholder->email,
+        ]);
+
+        $invitation = $owner->currentTeam->teamInvitations()->create([
+            'email' => $user->email,
+            'role' => 'employee',
+        ]);
+        $this->actingAs($user);
+
+        // Act
+        $acceptUrl = URL::temporarySignedRoute(
+            'team-invitations.accept',
+            now()->addMinutes(60),
+            [$invitation->getKey()]
+        );
+        $response = $this->get($acceptUrl);
+
+        // Assert
+        $user->refresh();
+        $placeholder->refresh();
+        $this->assertCount(0, $owner->currentTeam->fresh()->teamInvitations);
+        $this->assertCount(1, $user->organizations);
+        $this->assertContains($owner->currentTeam->getKey(), $user->organizations->pluck('id'));
+        $this->assertCount(5, $user->timeEntries);
+        $this->assertCount(0, $placeholder->timeEntries);
+    }
+
+    public function test_team_member_accept_fails_if_user_with_that_email_does_not_exist(): void
+    {
+        // Arrange
+        Mail::fake();
+        $owner = User::factory()->withPersonalOrganization()->create();
+        $user = User::factory()->withPersonalOrganization()->create();
+        $invitation = $owner->currentTeam->teamInvitations()->create([
+            'email' => 'firstname.lastname@mail.test',
+            'role' => 'employee',
+        ]);
+        $this->actingAs($user);
+
+        // Act
+        $acceptUrl = URL::temporarySignedRoute(
+            'team-invitations.accept',
+            now()->addMinutes(60),
+            [$invitation->getKey()]
+        );
+        $response = $this->get($acceptUrl);
+
+        // Assert
+        $this->assertCount(1, $owner->currentTeam->fresh()->teamInvitations);
+        $user->refresh();
+        $this->assertCount(0, $user->organizations);
+    }
 }
diff --git a/tests/Feature/RegistrationTest.php b/tests/Feature/RegistrationTest.php
index 23ab3426..4373809f 100644
--- a/tests/Feature/RegistrationTest.php
+++ b/tests/Feature/RegistrationTest.php
@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Tests\Feature;
 
+use App\Models\User;
 use App\Providers\RouteServiceProvider;
 use Illuminate\Foundation\Testing\RefreshDatabase;
 use Laravel\Fortify\Features;
@@ -38,10 +39,47 @@ class RegistrationTest extends TestCase
 
     public function test_new_users_can_register(): void
     {
-        if (! Features::enabled(Features::registration())) {
-            $this->markTestSkipped('Registration support is not enabled.');
-        }
+        $response = $this->post('/register', [
+            'name' => 'Test User',
+            'email' => 'test@example.com',
+            'password' => 'password',
+            'password_confirmation' => 'password',
+            'terms' => Jetstream::hasTermsAndPrivacyPolicyFeature(),
+        ]);
 
+        $this->assertAuthenticated();
+        $response->assertRedirect(RouteServiceProvider::HOME);
+    }
+
+    public function test_new_users_can_not_register_if_user_with_email_already_exists(): void
+    {
+        // Arrange
+        $user = User::factory()->create([
+            'email' => 'test@example.com',
+        ]);
+
+        // Act
+        $response = $this->post('/register', [
+            'name' => 'Test User',
+            'email' => 'test@example.com',
+            'password' => 'password',
+            'password_confirmation' => 'password',
+            'terms' => Jetstream::hasTermsAndPrivacyPolicyFeature(),
+        ]);
+
+        $this->assertFalse($this->isAuthenticated(), 'The user is authenticated');
+        $response->assertInvalid(['email']);
+    }
+
+    public function test_new_users_can_register_if_placeholder_user_with_email_already_exists(): void
+    {
+        // Arrange
+        $user = User::factory()->create([
+            'email' => 'test@example.com',
+            'is_placeholder' => true,
+        ]);
+
+        // Act
         $response = $this->post('/register', [
             'name' => 'Test User',
             'email' => 'test@example.com',
diff --git a/tests/Unit/Endpoint/Api/V1/ImportEndpointTest.php b/tests/Unit/Endpoint/Api/V1/ImportEndpointTest.php
index c53b2fde..5534cf14 100644
--- a/tests/Unit/Endpoint/Api/V1/ImportEndpointTest.php
+++ b/tests/Unit/Endpoint/Api/V1/ImportEndpointTest.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Tests\Unit\Endpoint\Api\V1;
 
 use App\Models\Organization;
+use App\Service\Import\Importers\ReportDto;
 use App\Service\Import\ImportService;
 use Laravel\Passport\Passport;
 use Mockery\MockInterface;
@@ -20,7 +21,7 @@ class ImportEndpointTest extends ApiEndpointTestAbstract
         Passport::actingAs($data->user);
 
         // Act
-        $response = $this->postJson(route('api.v1.import', ['organization' => $data->organization->id]), [
+        $response = $this->postJson(route('api.v1.import.import', ['organization' => $data->organization->id]), [
             'type' => 'toggl_time_entries',
             'data' => 'some data',
             'options' => [],
@@ -41,6 +42,14 @@ class ImportEndpointTest extends ApiEndpointTestAbstract
                 ->withArgs(function (Organization $organization, string $importerType, string $data, array $options) use (&$user): bool {
                     return $organization->is($user->organization) && $importerType === 'toggl_time_entries' && $data === 'some data' && $options === [];
                 })
+                ->andReturn(new ReportDto(
+                    clientsCreated: 1,
+                    projectsCreated: 2,
+                    tasksCreated: 3,
+                    timeEntriesCreated: 4,
+                    tagsCreated: 5,
+                    usersCreated: 6,
+                ))
                 ->once();
         });
         Passport::actingAs($user->user);
@@ -54,5 +63,27 @@ class ImportEndpointTest extends ApiEndpointTestAbstract
 
         // Assert
         $response->assertStatus(200);
+        $response->assertExactJson([
+            'report' => [
+                'clients' => [
+                    'created' => 1,
+                ],
+                'projects' => [
+                    'created' => 2,
+                ],
+                'tasks' => [
+                    'created' => 3,
+                ],
+                'time-entries' => [
+                    'created' => 4,
+                ],
+                'tags' => [
+                    'created' => 5,
+                ],
+                'users' => [
+                    'created' => 6,
+                ],
+            ],
+        ]);
     }
 }
diff --git a/tests/Unit/Endpoint/Api/V1/UserEndpointTest.php b/tests/Unit/Endpoint/Api/V1/UserEndpointTest.php
new file mode 100644
index 00000000..9cbe27a2
--- /dev/null
+++ b/tests/Unit/Endpoint/Api/V1/UserEndpointTest.php
@@ -0,0 +1,85 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\Endpoint\Api\V1;
+
+use App\Models\Organization;
+use App\Models\User;
+use Laravel\Passport\Passport;
+
+class UserEndpointTest extends ApiEndpointTestAbstract
+{
+    public function test_index_returns_members_of_organization(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'users:view',
+        ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->getJson(route('api.v1.users.index', $data->organization->id));
+
+        // Assert
+        $response->assertStatus(200);
+    }
+
+    public function test_invite_placeholder_fails_if_user_does_not_have_permission(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+        ]);
+        $user = User::factory()->create([
+            'is_placeholder' => true,
+        ]);
+        $data->organization->users()->attach($user);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.users.invite-placeholder', ['organization' => $data->organization->id, 'user' => $user->id]));
+
+        // Assert
+        $response->assertStatus(403);
+    }
+
+    public function test_invite_placeholder_fails_if_user_is_not_part_of_organization(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'users:invite-placeholder',
+        ]);
+        $otherOrganization = Organization::factory()->create();
+        $user = User::factory()->create([
+            'is_placeholder' => true,
+        ]);
+        $otherOrganization->users()->attach($user);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.users.invite-placeholder', ['organization' => $data->organization->id, 'user' => $user->id]));
+
+        // Assert
+        $response->assertStatus(403);
+    }
+
+    public function test_invite_placeholder_returns_400_if_user_is_not_placeholder(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'users:invite-placeholder',
+        ]);
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->postJson(route('api.v1.users.invite-placeholder', ['organization' => $data->organization->id, 'user' => $data->user->id]));
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertExactJson([
+            'error' => true,
+            'key' => 'user_not_placeholder',
+            'message' => 'The given user is not a placeholder',
+        ]);
+    }
+}
diff --git a/tests/Unit/Model/UserModelTest.php b/tests/Unit/Model/UserModelTest.php
index c0d72d37..228284cf 100644
--- a/tests/Unit/Model/UserModelTest.php
+++ b/tests/Unit/Model/UserModelTest.php
@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace Tests\Unit\Model;
 
+use App\Models\Organization;
+use App\Models\TimeEntry;
 use App\Models\User;
 use App\Providers\Filament\AdminPanelProvider;
 use Filament\Panel;
@@ -42,4 +44,47 @@ class UserModelTest extends ModelTestAbstract
         // Assert
         $this->assertTrue($canAccess);
     }
+
+    public function test_scope_belongs_to_organization_returns_only_users_of_organization_including_owners(): void
+    {
+        // Arrange
+        $owner = User::factory()->create();
+        $organization = Organization::factory()->withOwner($owner)->create();
+        $user = User::factory()->create();
+        $user->organizations()->attach($organization, [
+            'role' => 'employee',
+        ]);
+        $otherOrganization = Organization::factory()->create();
+        $otherUser = User::factory()->create();
+        $otherUser->organizations()->attach($otherOrganization, [
+            'role' => 'employee',
+        ]);
+
+        // Act
+        $users = User::query()
+            ->belongsToOrganization($organization)
+            ->get();
+
+        // Assert
+        $this->assertCount(2, $users);
+        $userIds = $users->pluck('id')->toArray();
+        $this->assertContains($user->getKey(), $userIds);
+        $this->assertContains($owner->getKey(), $userIds);
+    }
+
+    public function test_it_has_many_time_entries(): void
+    {
+        // Arrange
+        $user = User::factory()->create();
+        $timeEntries = TimeEntry::factory()->forUser($user)->createMany(3);
+
+        // Act
+        $user->refresh();
+        $timeEntriesRel = $user->timeEntries;
+
+        // Assert
+        $this->assertNotNull($timeEntriesRel);
+        $this->assertCount(3, $timeEntriesRel);
+        $this->assertTrue($timeEntriesRel->first()->is($timeEntries->first()));
+    }
 }
diff --git a/tests/Unit/Service/Import/ImportDatabaseHelperTest.php b/tests/Unit/Service/Import/ImportDatabaseHelperTest.php
index f0a03d9b..d1d156f2 100644
--- a/tests/Unit/Service/Import/ImportDatabaseHelperTest.php
+++ b/tests/Unit/Service/Import/ImportDatabaseHelperTest.php
@@ -51,21 +51,27 @@ class ImportDatabaseHelperTest extends TestCase
         ]);
     }
 
-    public function test_get_key_not_attach_to_existing_returns_key_for_identifier_without_creating_model(): void
+    public function test_get_key_not_attach_to_existing_is_not_implemented_yet(): void
     {
         // Arrange
         $project = Project::factory()->create();
         $helper = new ImportDatabaseHelper(Project::class, ['name', 'organization_id'], false);
 
         // Act
-        $key = $helper->getKey([
-            'name' => $project->name,
-            'organization_id' => $project->organization_id,
-        ], [
-            'color' => '#000000',
-        ]);
+        try {
+            $key = $helper->getKey([
+                'name' => $project->name,
+                'organization_id' => $project->organization_id,
+            ], [
+                'color' => '#000000',
+            ]);
+        } catch (\Exception $e) {
+            $this->assertSame('Not implemented', $e->getMessage());
+
+            return;
+        }
 
         // Assert
-        $this->assertNotSame($project->getKey(), $key);
+        $this->fail();
     }
 }
diff --git a/tests/Unit/Service/UserServiceTest.php b/tests/Unit/Service/UserServiceTest.php
new file mode 100644
index 00000000..79494844
--- /dev/null
+++ b/tests/Unit/Service/UserServiceTest.php
@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\Service;
+
+use App\Models\Organization;
+use App\Models\TimeEntry;
+use App\Models\User;
+use App\Service\UserService;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+class UserServiceTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_assign_organization_entities_to_different_user(): void
+    {
+        // Arrange
+        $organization = Organization::factory()->create();
+        $otherUser = User::factory()->create();
+        $fromUser = User::factory()->create();
+        $toUser = User::factory()->create();
+        TimeEntry::factory()->forOrganization($organization)->forUser($otherUser)->createMany(3);
+        TimeEntry::factory()->forOrganization($organization)->forUser($fromUser)->createMany(3);
+
+        // Act
+        $userService = app(UserService::class);
+        $userService->assignOrganizationEntitiesToDifferentUser($organization, $fromUser, $toUser);
+
+        // Assert
+        $this->assertSame(3, TimeEntry::query()->whereBelongsTo($toUser, 'user')->count());
+        $this->assertSame(3, TimeEntry::query()->whereBelongsTo($otherUser, 'user')->count());
+        $this->assertSame(0, TimeEntry::query()->whereBelongsTo($fromUser, 'user')->count());
+    }
+}