First, thanks for the link.
Second, AAAAAAAAHHHHHHH!!!
I've been looking for a "what groups are they in" Address Book solution since Address Book arrived. Lo and behold, it's been there all along:
http://www.apple.com/pro/tips/addressbook_groups.html
Anyway, this weekend's project is to update my 1,500-plus-name Address Book, so thanks for the well-timed tip/link. You probably just saved me about 5 hours of very tedious double-checking with my groups.