Unable to Comply, Building in Progress
So if Microsoft doesn't want to give you games to play with in your office, what's the next best thing to do.. as in the legal thing to do, without resorting to piracy? Well, you can try out first person shooters (FPS) such as Quake III Arena (Q3A) and Unreal Tournament 2004 (UT2K4).
Their demo versions are pretty comprehensive with Q3A focusing on deathmatch and UT2K4 with its myriad of team play options. The Onslaught mode in UT2K4 deserves special mention, since it involves playing in teams, capturing power nodes across a pretty big map. Create a path of captured power nodes to the enemy base and destroy the power node within that base to win the match.
Not into FPS? Well, if you can get your boss to spend a little on one copy of Starcraft... you'll get people yelling: "You want a piece of me, boy?!" or "My life for Aiur!", in no time. You can perform spawn installations of Starcraft (from a single copy of the game) for the purpose of having some localised multiplayer action.

When all else fails, get a PS2, a projector and a bunch of games, now that's what I call living, I mean, working. All puns intended.