I created my forums on wikidot
http://www.geetasks.com/forum/c-91340/discussions and that's were Beta testers discuss stuff.
I also plan to use Mailchimp to send out announcement about the next Beta being released, because many Beta testers are not reading the forums all the time. I have negative experience with BCC - it seems that many spam filters just kill those messages.
I wish someone had created an integrated Beta management service and I would gladly pay for it. Something that allows people to sign up (with instructions on how to submit device ID for ad-hoc builds), unsubscribe, get notified by email or RSS or twitter or whatever, a forum where people could discuss things, list of new features, list of released features, list of planned features, that kind of thing.