Chris Brogan tried the group idea with "Twitter Packs"... he implemented it quickly and dirtily using a Wiki though, so editing it runs into some obvious pitfalls (e.g., contention when a page is edited by more than one person)...

http://twitterpacks.pbwiki.com/

I toyed with the idea of implementing it as an actual standalone service without the wiki... but I have too many other "side projects" right now! :)

Here's my twitter account: http://twitter.com/mdesjardins