I believe that BuddyPress registration is integrated with WordPress', so if you create a user he'll be added to the WP users table. BP just changes the registration process a little, but in the user ends up in the deafult table for users in the database. But this is all on the same blog.
If you want them to use BuddyPress in a separate blog from the one they already have the account in (e.e. the parent blog), you'll need to research how to share the userbase between blogs in a multisite network, I believe there are some results about that in Google. I can't really help with that, I'm not familiar with it.