def METHOD_NAME(self, group, user):
    if 0 != self._check_member_accnt(user):
        perror('group', group+':', 'cannot add inexistant user "%s"'%user)
        return RET_ERR
    if self.get_primary_group(user) == group:
        pinfo("group %s is already the primary group of user %s: skip declaration as a secondary group (you may want to change your rule)" % (group, user))
        return RET_OK
    ag = self.member_of(user)
    g = ag | set([group])
    g = ','.join(g)
    return self.fix_member(g, user)