code: def add_group_user(self, group_id, user_id):
          
          data = {
              "id": user_id
          }
  
          response = self._perform_request(
              url='/um/groups/%s/users' % group_id,
              method='POST',
              data=json.dumps(data))
  
          return response
docs: Adds an existing user to a group.
  
          :param      group_id: The unique ID of the group.
          :type       group_id: ``str``
  
          :param      user_id: The unique ID of the user.
          :type       user_id: ``str``