Thank for all the replies guys, i'm looking into all your suggestions and found a few solutions to finding out what computer a user is on. I found an oddly simple method that involves going into the Computer Management window, then going System Tools/Shared Folders/Sessions and then finding the user and the corresponding computer next to it, though apparently it sometimes gives an IP instead of a host name. But you have to look through the list manually, so I suppose that doesn't help too much but does mean that I could gather this information somehow via command line maybe, or via Microsoft's PsLoggonOn library you can download.
If you have more i'll happily read them!