First, you may want to consider using the WP toolbar (which features full BP integration), because the BP BuddyBar will not continue to be supported in future versions of BP. (It will probably still work, but it won’t be actively upgraded.) Flip the Toolbar on by putting define( 'BP_USE_WP_ADMIN_BAR', true ); into your wp-config.php file. For more on customizing the Toolbar, a nice starting place is here: http://wp.smashingmagazine.com/2012/03/01/inside-the-wordpress-toolbar/
Second, if you are going to stick with the BuddyBar for now, you should just be able to create the dropdowns by putting nested <ul> elements into your main button <li>s:
echo '<li><a href="https://wordpress.stackexchange.com/forums/">Agent Forums</a>';
echo '<ul>';
echo '<li><a href="#">Item One</a></li>';
echo '<li><a href="#">Item Two</a></li>';
echo '</ul>';
echo '</li>';