my wife and I use the same iBook when I have it at home in the evenings. She gets annoyed at the fact that she can't listen to my music collection considering all the music I have is there, wheras her side doesn't really have anything. The only way she can really listen to anything is if I login, startup iTunes, and then she switches to her profile, launches her iTunes and my music shows up as a share.
Anyways, all that history aside, would this work for a multiple user, same mac environment:
1) iTunes music folder is \music\ (not in a user folder), and set to have read/write by anyone.
2) one time - make a copy of the xml file in the user folder of the user with all the music in iTunes, and put that into \music\xml file\.
3) have an application that will upon login of any user, copy the xml file from \music\xml file\ to \user\music\iTunes Music.....\
... at this point, the entire music library should be visible in any user's iTunes b/c they have a current XML file
4) to keep it current, anytime the XML file changes in any user's directory due to adding music, the script/app/whatever will copy the \user\music... xml file -> \music\xml file\...
I don't know how clear that is, but it all makes sense in my head at the moment. Would that work?