Ok – well this following change has already been committed into core code so will be in the next SP update. The core file to edit is
/simple-press/sp-startup/forum/sp-forum-support-functions.php
The function you need to find is sp_populate_support_vars() around line 137
A few lines into that function you will find the line of code:
$sfvars['forumid'] = $record->forum_id;
Below this simply add:
$sfvars['groupid'] = $record->group_id;
Finally, in your SP theme functions file in the small sort routine, change the $sfvars[‘forumid’] to $sfvars[‘groupid’] along with the appropriate Group record number.