BuddyPress.org

Yes. Make your adjustments in this file:
buddypress\bp-templates\bp-legacy\buddypress\groups\single\members.php

Create a template overload of that file first.

Try adding this under the member name link:
<h2 class="user-nicename">@<?php bp_activity_get_user_mentionname( bp_get_group_member_id() ); ?></h2>