DOOM and Command and Conquer (1995) are DOS games.
Warcraft III and Unreal Tournament are Win32 games.
These aren't even from the same era, so it sounds like you have a lot of planning to do. If you want to play all the golden oldies (DOOM, C&C, Quake, Duke Nukem 3D, etc)- then you can very easily do so through DOSBox and the Mac specific counterpart Boxer. DOSBox supports IPX over TCP emulation, so it's trivial to interlink a bunch of computers and enjoy all those old games together.
Games like Warcraft III and Unreal Tournament are considerably more tricky to coordinate between OS X and Windows (it's no longer a simple matter of installing it under DOSBox and everything "just works"). Warcraft III had a Mac version, but it's antiquated and won't install under 10.8. You could try to run it under WINE, but YMMV.
UT'99 is basically the same thing. You'll probably have to run it under WINE, or a virtual machine managed by VMware Fusion or Parallels Desktop (and neither product is free, unfortunately).
Basically, Windows games are a pain in the ass in this regard. DOS games aren't because you've got DOSBox, and once they're installed somewhere you can just copy over the config file and the entire C:\ drive folder to someone else's computer and you're done.
Of course, there's also modern day games like Borderlands 2 that are simply flat out compatible with OS X via Steam, and offer cross-system play... But everyone needs to own their own license (via Steam) for that to work.
So I really think you need to decide what you want to play, then figure out how you're going to make that happen so you're not wasting time when people show up. DOS games are by far the easiest. Windows games are significantly more involved, especially if you have to run them under Mac OS X.
-SC