WordPress MU

I would be interested in any suggestions to meet a use case for MU as follows:

- There will be a main site (WPMU), requiring login to view

- There will be 8 - 9 blogs created under it using: domain.com/blog1 approach not blog1.domain.com approach - these will also be private

- A base of up to 200 users will be split into 8 groups, each assigned to one of the blogs.

- All users can login to the main site, which will also provide access to the blog / group the user is assigned to.

- Users can only view the blog they are assigned to, not any of the other blogs.

- Ideally users will only need to log in once, such that logging in to the main site makes the link to their blog visible.

Could anyone point to any MU friendly plugins out there that provide for this level of configuring and granularity and avoid the need for more than 1 set of log in credentials.

I am happy to look at commercial and non-commercial options.

Thanks for any advice offered.

One plugin does it.
http://wpmudevorg.wordpress.com/project/More-Privacy-Options/

(orange box on the side, it's free)

You can tweak the privacy option when you go to Settings -> Privacy in the admin area of each blog.

And when you assign users to a blog, they can't access the backend of the other blogs. The privacy plugin has an option for only being allowed to view the blog if you are assigned to it.

the logging in once is default.

Apologies for the delay in responding. Thanks for that Andrea, much appreciated. I will check it out.