You have multiple Xboxs connected to one router. Whoever creates a game, you can join. Only people on that router can play with each other.
However, there is this program that tricks your Xbox into thinking it found a system link came whenever you join somebody's lobby on the program.
So after you create an account:
1. Join a lobby
2. Open System Link and there will be a game
3. Join that game
4. Have fun!
You can also host a game of your own which I recommend because most people only play that MLG, SWAT, and Snipers crap.
[Edited on 12.23.2011 9:52 AM PST]