Animal Jam has some fun mutliplayer games that you can play with your friends. To play a game with other Jammers, find one of the multiplayer game controllers, click on it, and a lobby popup will appear. For example, here is the lobby for River Race:
You can play these games as a single player, or wait for others to join the lobby. When the lobby is filled up or when you click the Start Game button, the game will start.
Here is a list of the Animal Jam multiplayer games:
Also, check out the Buddy Games for quick, short, two player games.