onInit () {
  this.state = { 
    players: {/* add players here when they join */},
    currentTurn: "" // sessionId of the current player
  }
}

onJoin(client, options) {
  this.state.players[client.sessionId] = {/* player data */}
}

onMessage (client, data) {
  if (client.sessionId === this.state.currentTurn) {
    // perform your action

  } else {
    console.log("only the current active player can send messages this time")
  }
}