Did you ask in the BuddyPress forum? BP uses its own registration.
Thanks Andrea, I’ll try there. But BP also precludes WP registration. Even when profile synch is disabled. Is there some way around using Buddy Press for a forum that works with WPMU 3.0?
Buddypress isn’t a forum, it just uses bbPress (see bbpress.org). Keep in mind, bbPress is being turned into a Plugin. Eventually.
Do you want a forum or do you want a social network? BuddyPress is a social network first and foremost. There’s a small section that has a forum area in it.
If you just want a forum, install a forum package.
I’m aware of situation lik eyou describe – but the answers and the best people to answer them – are all over in the buddypress support forum.