Members belong to a specific bp/mu site. If you have two mu installs then you have two independent membership groups. Never the two shall meet.
Users who are registered with blogA.yoursite.com do not have to be registered with blogB.yoursite.com. Blogs have independent membership lists as well.
Just because I’m a member of yoursite.com it doesn’t automatically give me any role in any blog on yoursite.com other than ‘subscriber’ to blog id 1.