public function subscribed_users(user_selector_base $existingusers) {
        $output  = $this->output->box_start('subscriberdiv boxaligncenter');
        $output .= html_writer::tag('p', get_string('forcesubscribed', 'forum'));
        $output .= $existingusers->display(true);
        $output .= $this->output->box_end();
        return $output;
    }