To do this achievement press the Select button
on your Xbox One controller and use the right stick to navigate down to where it says Multiplayer. Press
to select it and then select the first option (should say something along the lines of "Online Co-op". The game will then match you up with another online player. Once this has happened, complete a mission (they are dotted around the map and titled "Online Operation") along side the other player and the achievement should unlock.