This achievement is called "Together in the light" and is awarded for completing 5 quests with the same coop partners.
This can be done anytime after the prologue.
If you are starting a new game continue on single player until you get out of the hotel and into the shack on the beach.
After the cut scene pause the game with the double window button and the move tabs to the online tab.
Now ensure its either set to either friends only or public.
Now it's simply a case of inviting someone in and completing 5 quests. This early on quests are really easy and shouldn't take too long but this can be done at any other Point in the game.
Also note this must be done with the same people so it's probably best to do this with a friend on invite only to stop any randoms joining and then dropping out after 4 quests causing you to start 5 more.