This achievement requires that you complete a crossword puzzle with at least three other members of your team. Create a private game and invite your team members. They do not all need to join at the same time, be there for the entire puzzle, or even when the puzzle is completed. As long as they are shown below the puzzle in the players list you will unlock the achievement when it is completed.
NOTE: In order for all four players to unlock the achievement by playing a single puzzle, EVERY player must add the other three players to THEIR team.