BuddyPress.org

Just some basic questions: multisite (which is important as a different process)? registration steps completed? data in mysql for a failed user signup? (check _users->”user_status” and _usermeta->meta_key->”activation_key”)

I was pointing out the php file and functions – if you wanted see the flow of the signup. (wp_insert_user, setting user_status, generating and saving activation_key, sending activation email to user). You mentioned “My install of wordpress does send emails successfully, until I activate Buddypress (then it breaks).” BuddyPress uses the same WordPress functions to send an email – so nothing changes in that regard but maybe some weird oddity in your setup causing a null email address or some other unpredictable result.