Firstly you need to set the permissions of your Music folder so that every user you want to have access to it does. You may need to apply this to enclosed items.
Then make an alias of your Music folder in their user folder. Load iTunes and if it gives an error then there's probably a permissions problem.
A few things to note though: Firstly if you want a user to have access to all of another user's music but also have some other unshared music, create aliases of all the artists/albums/songs inside the shared music folder and put them in the other Music folders (in the correct locations). The quickest way to do this is command-option-drag. This also applies if you only want to share part of a library, in which case just create aliases of the artists, albums or songs.
If you just want to have one shared music library for every user, so everyone has exactly the same music, create a Music folder in /Users/Shared and move everyone's music into it (it may be easier to move a Music folder there and then modify it). Make sure it has read/write access for everyone. Then make aliases to it from every user's folder.