Well, this is the *WordPress* forum, not the *WordPress MU* forum. Everyone here knows a lot about WordPress, but not so much about MU (although some of us do.) If you want reallyreat information about MU, you need to ask over there.
That aside, "MU" stands for "multi-user" - which really means "multi-blog". WordPress.com is a WordPress MU setup. One installation of WordPress, many separate and individual blogs. You sign up for an account and you get your own blog.
If you want *one* blog, with *multiple* contributors to it, to be used as a CMS, then you just want WordPress. It does that just fine.