multi-blog membership

Anyone has experience dealing with this situation below?
We have a wpmu site running on yoursite.com. It's a members-only site.

There are a number of blogs we created, each with their own members:
yoursite.com/blogA
yoursite.com/blogB
etc.

How do we allow members of yoursite.com to automatically become members of blogA and blogB?

Also we do not want blogA or blogB members to access the content on yoursite.com.