The following assumes you want to keep all your iTunes music and movies on a shared folder and ensure new music is always stored on that folder, and that you want to share the iTunes library among multiple Users of your mac
First, you need to consolidate your iTunes Library to a folder you have chosen as the shared point.
Firstly create a folder to store your shared iTunes library, lets say
Users > Shared > Media. Then set iTunes to store everything at that folder from the iTunes advances preferences
Select the command consolidate library from the File Menu. This will copy all your itunes media to the folder and link the itunes library file to it
The files will now be copied while the
library file (the index as it were) will still be in the Users > Me > Music folder of your mac.
To get this file to be shared as well as well, copy it to Shared folder (Users > Shared > Media – or whatever folder you have set-up), start itunes with the alt key pressed and you will have the option to select the library file in shared
Select the library file you just copied and now both your music and the library in shared
Make sure that all users have read and write privileges for that folder and the libraryl file
Press the alt key in the other account to get iTunes to see the library file, now both accounts are using the same index. Make sure both Users itunes have the same location for the music folder set in their advances preferences.
You can now delete any old unused Itunes folder off your mac