The first two players to arrive will be player's X and O. X goes first, by clicking on any square. Any other players that join will automatically become spectators. If a player leaves (this take a few seconds to detect), the spectator who has waited the longest will take their place. When the game is finished, it will reset automatically after about 5 seconds. The winner stays in the game and the loser will be switched with a spectator.
NOTE: If no other players are available, you can open a copy of the web page up in a separate window/tab and play yourself.