How to share iTunes libraries between user accounts in OS X
You can change iTunes' default behavior to allow all accounts to use the same media folders.
If multiple users on an OS X system use iTunes, by default each will have a separate iTunes media folder in which music files are stored. This is convenient for privacy, but it will prevent users from sharing common music, which may be desired for household members or coworkers using the same Mac.
If you would like to share the same media files in iTunes among multiple users, there are a couple of approaches you can take.
One option is to share all iTunes music files between accounts, so while playlists and individual music organization for each account will be maintained separately, all music files themselves will be accessed from a common location.
To do this, open iTunes' preferences and go to the Advanced section. In here you will see a path to the iTunes Media folder location, where the program currently stores files.
Now use your mouse to highlighted the path text in blue, and then right-click the selection and choose "Reveal in Finder" from the Services contextual submenu. This will show the iTunes media folder in the Finder, which you can move to a common location accessible from all user accounts. Some options for this include the "Macintosh HD > Users > Shared" folder, or a secondary hard drive (external or internal).
With the folder moved to this location, click the Change button next to the media folder path in iTunes, and select the media folder in its new location. Update the iTunes preferences similarly for other accounts, and now all should use the same iTunes folder.
Keep in mind that iTunes may report some songs missing for some user accounts after changing their iTunes media locations, so in these cases you will have to re-import the songs by dragging them from these accounts' old media folders to the iTunes window or to the new media folder. Alternatively you can manually drag the contents of each user's old iTunes Media folder to the new one, to consolidate it in this new location (you will have to manually manage duplicate files when doing this).
A second approach to this is to decentralize your iTunes media folders, so you can maintain a separate private music collection, but also include music files from public folders, external drives, and other commonly accessible folders. To do this, instead of changing your iTunes Media folder location, leave it as-is and uncheck the option to "Copy files to iTunes Media folder when adding to library file" that is located under the media folder path.
With this setting unchecked, new songs you import from CDs or download from iTunes will be placed in your private collection in your user account; however, you can also drag a music collection folder from a shared drive to your iTunes window, and iTunes will add the files to your library but keep them in their current location instead of copying them to your private collection.