I was going to set one up for my football club.
Post our newsletter, send out weekly emails for duties, rosters, games, events, maintain up-to-date contact details, etc...
until I realised that to access it you need at least a trial / free @mac email!
This limitation I think will cripple the Groups concept.
Not everyone will have a User ID for iTMS, or want to sign up for yet another log in account. I've kept the group name, in the hope that in the future they might open it up to allow group members (but obviously not Group Owners) to join with any email address.