You can assign some property to a room to represent the state of the game i.e. started or not. For example you can set "isGameStarted" property to a room using updateRoomProperty() method. Whenever you are starting a game just set that porperty to true. Now Whenever a new player is going to join a room, you can use our matchmaking API joinRoomWIthProperty() to join a room which has this property set as false. When the game is over, revert back the the property to false value.