Set them up an Apple ID each through their computer account.
Move the current iTunes library to the shared folder in the users folder. (macintoshhd/users/shared)
Do a get info for the new music folder and ensure it ignores ownership, all users must be able to read and write to the folder else you get strange errors when downloading.
Point each accounts iTunes library to the new shared folder using the advanced section of the preference pane for iTunes.
While there untick the copy files to iTunes media folder (failure to do this and you end up with 4 of each track at the next step).
Starting with an empty music window in iTunes import all your music stuff back into iTunes for each account. When clearing, if it asks to trash files DON’T!!
Presto everyone gets to share all the music.
As tracks get added by others from the store you can import into each user account by selecting the music folder using the import command (keep the copy files to library box un-ticked unless you have files on your computer you wish to move to the library).
Redeem vouchers as normal for each user account.
This will simplify back-up as there is only one large folder to do for the music and films etc.