Help: multi-user blog?

[eluser]iDenta[/eluser]
Hi all!

I'm a total newbie, that started playing around with CodeIgniter by building the 20 min blog, with no former php knowledge except a little Wordpress modifying. After reading the docs and forum i have managed to add categories, timestamp, delete and edit posts, comment count and so on.

Now i want to try to make it possible to manage multiple blogs, so that a user can sign up, and a "new" blog is created under user.domain.com or domain.com/user, and keeping everything in one DB and one CI install, like Wordpress MU or Movable Type.

But i really don't know were to start, so i would really appreciate any pointers, links, forum posts or anything that can help me accomplish this kind of thing.

Thanks!