Users and workstations are just objects in AD. It's the connections that count, per the MS site for WIN2K3:
"Similar to previous Windows Server licensing models, one server license is required for each copy of the server software installed. In addition, a Windows Server 2003 Client Access License (CAL) is required for each user or device (or combination of both) that accesses or uses the server software."
So, your server needs a license, each workstation needs a license (usually the desktop OS license will suffice) and each user needs a license. Unless if you are running a specialized web connection or virtualizing, you should be covered by the above.
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