WordPress for Group Site

Dear All,
I am getting so near yet so far with WordPress from creating a group site.

It is for a research site, where different volunteer groups can post reports solely to their group members.

The site consists of a set of pages, which are viewable to all.

Each user should be assigned a role/group which will be the rest of their volunteers whose group they can post to, and whose posts they can read.

Uon logging in, they should be taken to their group homepage which is basically where all the blog posts from their group category are displayed. They can then comment on these, or contribute a new post of their own.

I have installed role-manager, category visiblity plugin but with no success, and just leaving me more confused.

I would appreciate any help.

Many thanks,
Giles