Multisite – restrict blog/site access to specific users

I have a situation in which the client wants to have a main site for public viewing, and then 3 private ‘areas’ that will be accessed by 3 different groups. This will not be for admin purposes, but for users of the site.

I activated the multisite option and it generally seems to be working fine.

What I really want to do is have users come to the site, view the pages that are publicly available, login and have access only to the pages that they are authorized to view.

I realize that this can be done on a page by page basis, but I was hoping it could be done on a ‘blog’ or ‘site’ basis. I want it to be relatively painless for my client to add new materials, and I want the users to be able to login one time and access everything they are authorized to view, without having to login again.

I have also looked over a few plugins but none of them seem to provide exactly what I’m looking for.

I am thinking that perhaps I’m not using the correct search terms.