You will need to have the Palace client in order to play.
The rules of Quidditch are:
There will be 3 chasers, 2 beaters, and 1 seeker on each team. The referee will "flash" Quaffles, Bludgers, and Snitches out on the Quidditch Field in Palace. Whichever chaser will be able to "grab" the Quaffle first gets the points. You may only "grab" the ball that corresponds with you. If the Beaters "grab" a bludger, they take away a point from the opposing team. The snitch will be worth 150 points and it will end the game. If you are not a player, but still want to play Quidditch, you can watch the games on Instant Palace, and if somebody doesn't show up, you can play for them.