two answers as I am unshour to what you want to do:
Setup homeshareing or:
1) QUIT not just Close iTunes if it open
1) Make a folder on the shared disk
2) Move the "Users/username/Music/iTunes folder you want to share with all the devices to the folder on the shared disk
3) Make that folder accessable by all the users you want to access it.
4) Hold the "Option/alt" key down and click iTunes and it will ask if you want to "Choose a Library"
5) click "Choose a Library" and choose the one on the shared disk
6) Do this on evey meachine you want to have access to this library
8) REMBER THAT IF YOUR NOT CONNECTED TO YOUR SHARED DISK THEN YOU WILL NOT BE ABLE TO ACCESS YOUR LIBRARY but you can choose librays at any time by using step 4 and you can have as meany librarys as you like.
I hope this answers your question
Sorry for any bad spellings I am dyslex......