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