docs: def get_group_id(self, user_id):
          """
          获取用户所在分组 ID
  
          详情请参考
          http://mp.weixin.qq.com/wiki/0/56d992c605a97245eb7e617854b169fc.html
  
          :param user_id: 用户 ID
          :return: 用户所在分组 ID
  
          使用示例::
  
              from wechatpy import WeChatClient
  
              client = WeChatClient('appid', 'secret')
              group_id = client.user.get_group_id('openid')
  
          """
          res = self._post(
              'groups/getid',
              data={'openid': user_id},
              result_processor=lambda x: x['groupid']
          )
          return res