Hitting it Off Achievement
Hitting it Off10
Play 3 quests with a friend (Campaign only).
Once you’ve loaded your game, press and tab across to "Online." Now select "Invite Friends" and choose a friend to join your game. All you have to do now is complete three missions of any sort - side missions, Outposts, Prepper Stashes - and the achievement will unlock.
All three quests do not have to be in your game; you can do them one on your file and the other two in a friend's game, for example.