To add a little detail:
Move one library to the shared drive (mine is): Mac HD>Users>Shared>iTunes>iTunes Music.
Set each users preferences to the shared folder: iTunes Preferences>Advanced>General and change the iTunes Music Folder Location to: Macintosh HD:Users:Shared:iTunes:iTunes Music
You can then add the other users music to the shared library. They will need to move their library to the shared folder. Then, from an iTunes account using the shared library, select File>Add to Library and select the other users music folder.