Some NAS drives will do it automatically for you, the files will simply show up in your iTunes library for you - its called a UPnP AV media server. I'm guessing your NAS drive doesn't have this function as if it did the files would already be in your iTunes library on any computer connected to the network.
The other way to do it, is simply add the files to your iTunes library. In iTunes, click on 'File', then 'Add to Library' and then in the box that opens just navigate to the NAS drive and the files your want to add and then iTunes will import them into your library. Before you do this, make sure you have gone into the preferences and told iTunes to not copy the files into iTunes library when importing otherwise it will add them to the music folder on the computer as well. To do this, before importing the files like I've said above, go into the preferences. In the Advanced tab you'll see a check box next to the 'Copy files to iTunes Music folder when adding to library' - make sure there isn't a check in this box. Once that's done, just add them and all the computers will be able to access the same files on the NAS drive at the same time whether you need them to or not. Hope this helps...