A pretty simple achievement. Simply join or host a game, get a few people playing, and run through 5 quests. They can be side quests or main quests. The nice thing about this achievement is that they don't have to be in your particular location in the story. You can play missions you've already done, or missions that are way ahead of your story progression. As long as you're playing co-op, and complete 5 quests, the achievement is yours.
Note: The description of the achievement can be a tad misleading. You don't have to complete 5 quests with the same partners, it can be just a partner. It's a small point, but I feel it's important enough to be noted, so you're not frantically searching for that third person.