How do you play assassins creed brotherhood online

First, you're going to want to download the free DLC, it adds a game type, map, and makes the load go a lot faster.
Second, select "Multiplayer" on the main menu.
Third, watch the video, then go to play>Introductory Session.
Fourth, if you want to familiarize yourself with the multiplayer more, then go to "User Manual". If you are ready to play, then go to "Play now", "Ranked Match", or "Player Match".

Player Match-You can invite friends or your XBL party to play and select a game type. You will then be matched up to get 6-8 players in a game. Can invite friends

Play Now-this will send you to a player match with any game type. Cannot invite friends

Ranked Match-Just like a player match except that you cannot play against friends. Can invite friends