Prerequisites: [1] You need to be on a LAN for that to work. [2] Every one of you should use the same (and current) version of iTunes. Make sure you've all upgraded to iTunes 4.6.
Go to: iTunes > Preferences > Sharing >
click the "Share my music" checkmark, and checkmark all the playlists that you want others on the LAN to "share" (or listen to from your machine), and give your shared music a name.
If you do activate shared music for your machine, everyone else on the LAN can access it. So to access others' libraries, they need to follow the above procedure. (Or you can activate Sharing on their machines while they have their backs turned...)
EDIT: The above procedure works for sharing your library among multiple computers, not just one computer with multiple users. So follow the link 2 posts down.