BuddyPress.org

I have a WPMU/BP sub-domain network configured with nine (9) different sites, as of right now:
active.mysite.com
business.mysite.com
cars.mysite.com
I’m using the Networks+ plugin and so ideally users would have the ability to purchase blogs off each of the communities. The way it looks right now is that all BP features, including users, are shared throughout the network. What I’m wanting to do is isolate groups to their specific community. So for instance:
active.mysite.com has the following groups:
fitness, running, hop-scotch
business.mysite.com has the following groups:
advertising, marketing, finance
cars.mysite.com has the following groups:
audi, range rover, kia

The way things are right now is that if you’re on the Cars site you can see groups from the Active and Business sites. What I’d prefer is for the groups to be only seen on that site. So if I have 10 groups on Cars you only see those 10 groups. Where it gets even more complicated is for users to only register once, join their groups, and navigate between the communities as their membership allows. What I’m trying not to do is have separate database installs of WPMU / BP for each community as I’m thinking this will force users to have to manage multiple accounts and passwords. So I’m wondering if I’m forced to tear down and install a single instance of BP with 100’s of groups nested under it or if there’s a way to isolate everything except the user accounts to specific communities or networks?

Any responses appreciated