workgroup or domain configuration?
if its domain, the easier way would be the Users tab under Task Manager. not 100% sure if this works with a workgroup environment. my inkling is "no", but it's easy enough to check.
as far as from the command prompt, you could run netstat (and a flag such as -a or -n or whatever, depending on what your looking for info-wise. -a should just show all connections, including listen ports). if you know the NetBIOS name of the clients in question, youll be able to ID who that connection belongs to. keep in mind that netstat usually lists more info that what i think youre looking for, depending on the flag/parameter you use.
i'm sure there are specific tools for this purpose as well, as the primary problem is that it's very hard to tell if a client is still actually connected without the server probing or asking in some way. if the kerberos ticket hasn't expired, it will look like a user is still logged on. outstanding tickets, i believe, arent really a help in assembling a list of users in the way you're looking for (who is actually logged into that server at that moment in time).
honestly, i would wait for additional input on this from other users. server OSs arent really my strong suit. i just thought i'd throw out a couple ideas in the meantime.