bbPress.org

If you’ve cracked the code for the submenu, then groups are a buddypress thing, so their forum might be better.

But from total ignorance try….

If you’ve phpmyadmin access, you could set up a user for a group, and see what changes in the usermeta, and then use this to create your filter

eg
if say you found that buddypress_group was what was used for groups, you could set the group using

$current_user = wp_get_current_user();
$group=get_user_meta( $current_user , 'buddypress_group', true);

then dependant on group

if ($group==22) then...
if ($group==23) then....