This is really two questions.
1. When someone registers a username/password in BuddyPress, I'd like them to be automatically assigned to the indefinite "Basic" (free) subscription, but I think I'm not supposed to set a default user registration level?
2. Regardless — when someone has a subscription, there don't appear to be any "sign up" buttons on the OTHER subscriptions on the /register page. See screenshot in the next post.