Group admins unsubscribing should pick other admin 1st
A group admin shouldn't unsubscribe, and should assign another user as group admin first if there is no other...
So when clicking the button, if you are admin and the last user (something like if (group::is_admin($group_id,api_get_user_id()) && group::users_count($group_id)==1), you'll have a popup that says:
"You are an admin for this group, please select another admin before you unsubscribe".
Now the tricky part is you'll have (as admin) to have an interface to select another admin...