From the start menu press on Start Game and you will appear on the ship the 'USS McKinley'. Once aboard turn left and an onscreen prompt saying 'Radio Room' will be visible. Walk over to it and press the button to start Matchmaking. From here you will be able to find other players online by pressing on 'Find Other Players'.
Alternatively, you may invite people from your party or friends list into the game when aboard the USS McKinley.
Complete a single mission with three others and you will unlock this achievement. The boss missions at the end of each act are generally the shortest missions.