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....