https://api.slack.com/methods/groups.invite

def invite(self, group_name, user):
        """ https://api.slack.com/methods/groups.invite
        """
        group_id = self.get_group_id(group_name)
        self.params.update({
            'channel': group_id,
            'user':    user,
            })
        return FromUrl('https://slack.com/api/groups.invite', self._requests)(data=self.params)