fix: DEV-3131: Fix user creating API (#2801)

label_studio/tests/users.tavern.yml

@@ -1,5 +1,5 @@
 ---
-test_name: test_ml_backend_creation
+test_name: test_users
 strict: false
 marks:
 - usefixtures:
@@ -21,3 +21,25 @@ stages:
     url: '{django_live_url}/api/users/{user_pk}'
   response:
     status_code: 200
+    save:
+      json:
+        org_pk: active_organization
+- name: stage
+  request:
+    json:
+      email: test_user@heartextest.com
+      username: test user
+      active_organization: !int '{org_pk}'
+    method: POST
+    url: '{django_live_url}/api/users'
+  response:
+    save:
+      json:
+        new_user_pk: id
+    status_code: 201
+- name: stage
+  request:
+    method: GET
+    url: '{django_live_url}/api/users/{new_user_pk}'
+  response:
+    status_code: 200

label_studio/users/api.py

@@ -128,6 +128,10 @@ def list(self, request, *args, **kwargs):
     def create(self, request, *args, **kwargs):
         return super(UserAPI, self).create(request, *args, **kwargs)
 
+    def perform_create(self, serializer):
+        instance = serializer.save()
+        self.request.user.active_organization.add_user(instance)
+
     def retrieve(self, request, *args, **kwargs):
         return super(UserAPI, self).retrieve(request, *args, **kwargs)