code: def rejoin(self, group_id):
          
          url = utils.urljoin(self.url, 'join')
          payload = {'group_id': group_id}
          response = self.session.post(url, json=payload)
          return Group(self, **response.data)
docs: Rejoin a former group.
  
          :param str group_id: the group_id of a group
          :return: the group
          :rtype: :class:`~groupy.api.groups.Group`