The iPod is able to play purchased music from two iTunes accounts, based from two different computers. Just select "Manage my own playlists" like the previous post suggested. As long as the computer you use is authorized on the account at the time you put the song on the iPod, the iPod ought to be able to play it.
I know this isn't quite what you had in mind, but this may make things easier than having everyone buy music on one computer and one account.
My wife and I both have separate iTunes accounts on our separate Macs, and have put our purchased music on the iPod in this manner.
Oddly enough, this doesn't seem to work the same way for Audible audiobooks: we have two Audible accounts now as well, and although my books play without a hitch, she just recently signed up for a separate account and the iPod doesn't want to play her audiobooks. I'm not sure if that's a limitation of Audible, or if the iPod can only store three decryption keys, or what...