To be specific, you should state your distro.
On RH9 and Fedora, the root user can create as many users as he/she/it/they like(s). There is usually a user managment tool.
For eg. I have three users (including root) who are:
"indigo-prime" specifies the computer and "subether" is the net.
There is also a computer called "Scene-23" and a handheld called "Q-Indigo". I havn't thought of having several domains on the same computer ... but I guess it would be easy if there were several ethernet cards or, possibly, one could apply aliases at the server level so that mail to simon@indigo-secundus (say) will also go to simon@indigo-prime.
But why would you want to?