Thanks for Helping Out, Bud!15G -
Finish your first co-operative quest (local or online).
As the achievement states, this can be done in either online or local co-op.
If you're doing this in online co-op, you can either host the mission or join along if your friend is the one that starts it. To join a friend, hold
when the notification appears. If you miss the notification, bring up the camp menu
and click "join" on the far right side.