def get_group_by_group_id(self, group_id):
        """ return group with given group_id (return None if doesn't exists) """
        group_id_base64 = base64.b64encode(group_id).decode('ascii')
        groups = self.m["groups"]
        for g in groups:
            if (g["group_id"] == group_id_base64):
                return g
        return None
return group with given group_id (return None if doesn't exists)