You can either add someone who is on your friends list already to your group, and play a game, or you can do something a little different and guarantees that you get this achievement.
Play a game. Before the game ends, add any player on your team to your friends list, and then invite them to your group. The "Stay As Team" feature works as well. As long as the person you just added to your friends list joins the group, and go into another game, the achievement will pop.