You can tell google groups to send emails. Not sure if you can set them to private. Obviously you can do a private install of a forum and restrict access.

You can make as private as you want, but if a message is only for a part of the team, or only like two people, a forum doesn't help.

You can usually make certain forum threads that have specific member lists, granted it's not as 'ad-hoc', but it'd be reasonably easy to add the features to a forum.