How to share an iTunes library with other user accounts on the same machine?
I was going to start explaining you what you can (and can’t) do or how hard was this going to be, but a single google query lead me to this excellent thread an Apple forum.

I’m going to quote the relevant part, but I suggest you read it all because there’s more information there and more “why” questions.

We assume you have copied all the music to a Central location (be it a machine or a NAS drive or whatever), so somewhere in the network or your harddrive all your music files (library) are accessible to your wife and you (and theoretically both can write to that location, important when adding files). 

It doesn’t matter if they are in different computers or just accounts.

With that in mind: 


Each future user of the library should launch iTunes with the Option key held down and Create Library. (A blank one). 
With this new iTunes window open, go into iTunes Preferences and UN-check "Keep iTunes Music folder organised" and "Copy files to iTunes Music folder when adding to library".
Then, they must add the music to the Library, selecting the tracks from the folder where all the music is stored (the common place mentioned above), which will import all the tracks into this new Library without copying the files again (hence why step #2).
From this point they can create their own playlists and stuff and they can even check the things they unchecked in step #2, in case they decide to add new tracks (see drawback), you’ll want to have them in the central location. 


From this point they both have a Library.XML (stores playlists, tracks location, etc.) and that library.xml points to the physical mp3/4 on the “common location” (tracks are not duplicated as you requested).

DRAWBACK (and the fact that there’s no better solution):

If either user of the library adds new music, regardless of the “keep music organized” and “copy files” checkboxes, the other user(s) are not going to see the changes. The reason being that they are modifying their local copy of the library “xml” (which is where all the playlist and stuff is stored). 

If USER1 adds 1 track, USER2 will have to find the track in the library and manually add it to his/her own library. 

As you can see, iTunes was not really designed for multiuser.

Go ahead and read the post, it will give you more pros/cons and maybe other ideas. 

EDIT: one way to know “what is new” in the library folder (where the music is actually stored) would be to create a smart folder that shows files added in the last “xx” days (for example). :)