In Multi-player or Nintendo WFC mode in Free For All or Team Conflict, wait for a game in "Shanty Town". Go to the right side of the building on the far left. Behind the building is a window with a barrel underneath. Jump onto the barrel, then jump onto the window so that all you can see is the roof of the building. Crouch down while standing in the window and climb through. You will have access to behind a few buildings. There are three ways to exit. Either die, exit the game, or switch teams on Team Free For All.