Instructions
Press "Up" on the Xbox 360 controller control pad after starting up the game to bring up the mobile phone.
Press the "A" button to bring up the menu.
Scroll down the list of options using the control pad and press "A" while highlighting "Multiplayer."
Scroll down to "Party Mode" on the next set of menus and press "A" to play with friends. When the multiplayer game starts press the Xbox guide button to bring up a window. Select "Friends" to view friends and invite them to the game.
Play with other Xbox Live users not on your friends list by selecting "Play Match" from the "Multiplayer" menu. This brings up the list of game modes. Choose "Custom Match" and then "Free Play."