The tracks will be copied from their original locations to the i Tunes Media folder path, which should reside on the networked hard drive.
Music from other computers can be imported via a network connection to those computers or simply copied onto an external hard drive and imported from there.
Once this has been done, you will be left with a single, master i Tunes library containing all of your content which will be stored on the networked hard drive.
You can also find instructions in the Transferring your i Tunes Library article on how to move your library over to the networked hard drive in the event that you want to be able to access it from more than one computer.
You will find that your i Tunes library and music collection is much easier to manage and maintain in this scenario, both within i Tunes itself and also for the purpose of synchronizing to your various media devices.