I am trying to allow each member that signs up on my buddypress WPMU site to become a global member but then to allow each subsite to allow global or new members to register or join that membership.

If someone goes to a specific site and attempts to sign up, they are taken to the main site. I want them to not only be added to the database for the main site buddypress install but should be added as a member of that specific site from which they sign up at.

Maybe I am not sure the concept of how this works, but I am very interested to get suggestions/solutions.

note, only the main site runs a buddy-press install at this time.