Share your music across a network
To share your music across a network, you first need to decide where the music itself will be stored. Choose which computer will hold the music, then open System Preferences on that computer and enable Personal File Sharing in the Sharing pane. Details on enabling file sharing on OS X can be found on Apple's support site at these links:
Once that's done, go to another computer that you'd like to access the music from and go the Finder. To connect to the shared computer, follow the directions on Apple's support site at:
Connecting to shared computers and file servers on a network
When you connect to the remote machine, it's easiest to log in with your default admin username and password, since that will let you access any hard drive on the machine, and thus let you store your music anywhere you want on the machine. Once you're connected, you can open up PowerTunes and set up your machine to use the remote library/music.
If you have an iTunes library set up on the remote machine that you want to use on your machine, all you have to do is add the library from the network drive to PowerTunes just like you would any other library. Note that you must also be able to access the media folder that this library uses, which is also most likely on the same machine. If the library and media folder are located on the same hard drive, then this is easy. If the media folder is on a different hard drive from the library file (e.g. the library file is on the internal drive but the music itself is stored on an external drive attached to the computer), then you will need to make sure that your machine can access both drives over the network.
If you only want to use the media folder on the remote machine, but have a separate library for your local machine, then you can either a) create a new library and use the "Choose media folder" option in the setup sheet to point it at the media folder on the network drive, or b) switch an existing library's media folder to point to the media folder on the network drive.
Important: If you are using a single library from multiple machines, and you open that library from both machines at the same time, you may end up getting errors reported by iTunes when it tries to save changes made to the library file. PowerTunes does its best to detect and warn before opening a library that's already open on another machine, but there are some cases that PowerTunes can't catch, and of course PowerTunes can't warn you if PowerTunes itself is not open. If you do end up with a library open on more than one machine at a time, your library should be safe, but you will probably lose any changes that you made from one of the two machines. When using this setup, do your best to quit iTunes on one machine before opening it on another machine if both machines are using the same library.