If you lack players, you can use a deck as an additional player by playing cards directly from it. This adds a random but sometimes hilarious element to the game.
In normal rounds you just take one card face down from any deck and add it to the mix. For Bonus Rounds make sure you set up one deck without red-bordered cards to draw the random cards from.
If the deck wins a round the round's Judge loses a point (if they have any).