Have two players sign in and proceed to the Campaign or Firefight lobby.
While in a Campaign mission or a Firefight game, press A for the amount of players that are going to join.
Quit the current mission/game.
Start the game again, but this time press A for the third player a second after the final beep of the countdown. For 4 players, do the same, but press A for the fourth player a split-second after the third player presses A (just before the game says "Setting Up Game").