Display a list of registered (group) members, as you join a private forum
 

Apologies if this is in the wrong section. If so, please move it.

I don't know if this exists already, or if it could cause a performance hit, but I am looking to:

Display a list of registrants for a particular sub forum, when someone joins it.

We have a forum with about 20 private sub forums, for various volunteer committee groups. I have each sub forum/committee assigned to its own VB usergroup. We simply check the user records for additional usergroups, to determine which ones they will be granted access to. This also causes a "rank" to display their committee/group affiliation on all posts. None of them have more than 10 members.

What I'm hoping for is a simple way to show the current list of group/forum members, when someone joins one of these sub forums. (but not for the public forums, because the list would be thousands of people)

I searched for hours and couldn't find anything that looks like it would fit the bill. Have you seen something that would do this, or does it sound like something that would be practical to program and hook in somehow? (without becoming a problem for performance)

Thanks for any guidance!
Rhody