code: def add_user(self, user, is_admin=False):
          
          users_count = self.users.all().count()
          if users_count == 0:
              is_admin = True
          # TODO get specific org user?
          org_user = self._org_user_model.objects.create(
              user=user, organization=self, is_admin=is_admin
          )
          if users_count == 0:
              # TODO get specific org user?
              self._org_owner_model.objects.create(
                  organization=self, organization_user=org_user
              )
  
          # User added signal
          user_added.send(sender=self, user=user)
          return org_user
docs: Adds a new user and if the first user makes the user an admin and
          the owner.