protected function add_userids(array $userids) : userlist_base {
        $this->set_userids(array_merge($this->get_userids(), $userids));

        return $this;
    }
protected function add_userids(array $userids) : userlist_base {
        $this->set_userids(array_merge($this->get_userids(), $userids));

        return $this;
    }
protected function add_userids ( array $ userids ) : userlist_base { $ this -> set_userids ( array_merge ( $ this -> get_userids ( ) , $ userids ) ) ; return $ this ; }
Add a set of additional userids.

@param   array   $userids The list of users.
@return  $this
Add a set of additional userids .