Unify and Synchronize Your iTunes Libraries (from MacDev)
Sharing iTunes
For iTunes, this is relatively simple. Make sure iTunes is closed. Now, move your existing iTunes folder (the one you want to share -- located at /Users/[username]/Music/iTunes) to /Users/Shared.
Select the iTunes folder that is in the Shared location and Get Info (Command-I) on it. In the Ownership & Permissions section, change everything to Read & Write, and click "Apply to enclosed items".
Next, make an alias of the folder, move it back to your ~/Music folder, and remove the "alias" string from the name. Repeat this step for all users that you want to have access to this shared library. You will need to delete their existing iTunes folder (which should be empty -- move any music to the Shared folder).