If you've been using your WordPress installation in a single-user fashion and have been looking to get more people in on the fun, you'll probably want to update to the WPMU edition of the popular blogging engine. It's not the easiest thing to accomplish, so the WordPress Arena site is here with a guide to walk you through each step of the way.
[WPMU] is most famously used for WordPress.com where it serves tens of millions of hits on millions of blogs each day. Many bloggers interested in using WordPress MU already have a WordPress blog. Unfortunately, I think it is not possible to turn WordPress installs into WordPress MU installs directly uploading new files and upgrading.
The post includes a 17-step process you'll need to follow to get your current WordPress installation migrated over intact (with a few sub-steps along the way). At the end, there's a brief tour of the new panels you'll have for things like the blogs, users, themes and administration.