Move your "iTunes Media" folder to the "/Users/Shared" folder or to your external drive. You may have to copy it, I can't remember now.
Then set the location of that folder under the advanced tab of the iTunes preferences for each user.
You also might have to use the "Add to library" command on the "iTunes Media" folder for each user. Again, I don't remember, I'm afraid, but it should do no harm. IIRC, iTunes is clever enough not to make duplicate entries in itself.
Once you have checked that all users are able to access the shared folder and play their music, if you copied (rather than moved) the "iTunes Media" folder, you may now delete the old one, leaving the other iTunes files and folders in their normal places for each user.