code: def link_user_to_group(self, user_id, group_id, headers=None, endpoint_url=None):
          
          endpoint_url = endpoint_url or self._endpoint_url
  
          url = endpoint_url + '/groups/' + group_id + '/link/' + user_id
          headers = headers or self._default_headers()
  
          response = requests.post(url, headers=headers)
  
          return response
docs: Links a user to a group
  
          :param str user_id: identified user's ID
          :param str group_id: group ID
          :param dict headers: custom request headers (if isn't set default values are used)
          :param str endpoint_url: where to send the request (if isn't set default value is used)
          :return: Response