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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def rejoin(self, group_id):
 """Rejoin a former group. :param str group_id: the group_id of a group :return: the group :rtype: :class:`~groupy.api.groups.Group` """

        url = utils.urljoin(self.url, 'join')
        payload = {'group_id': group_id}
        response = self.session.post(url, json=payload)
        return Group(self, **response.data)