1. How to send message to chosen player?
   I am going to use foreach to check all clients in the room with chosen player client ID. But i think it is bad solution.
2. I don't know one client can connect to many room or not.
   Because as i understood, these two players will be moved to Game room that only has 2 players. But i still want to keep their name and status in Lobby screen.
   What is best solution?
   Thanks you so much.

yes, clients can join multiple rooms. each room connection has its own websocket connection.

you'd need exchange messages between client and server during the Lobby room.

• sending messages from client to server
• onMessage in the client-side
• onMessage in the server-side

once your custom "handshake" between two clients has established on Lobby handler, you can send a "dule code" to the two players, and make the players join the same "dule" session.

client.join("dule", { code: "bothPlayersShouldJoinThisCode" });

on Dule room handler, you should only allow clients with the same dule code do join toguether:

maxClients = 2;

onInit (options) {
  this.code = options.code;
}

// on dule room handler
requestJoin(options) {
  return this.code === options.code;
}

I haven't tested the code, you can play around and modify as you see fit! Hope this helps, cheers!

// When a client sends a message
    onMessage (client, message) {
        switch (message.action) {
            //A send request to invite C to make a duel
            case 'request_dule':
                let guestId = message.data.guest_id;
                let guest = this.state.players[guestId];
                //find a client has id equal with C's client id
                for (let i = 0; i < this.clients.length; i++) {
                    if (this.clients[i].id == guest.client_Id) {
                        //send request to C's client and wait C accepting
                        this.send(this.clients[i], {action: 'request_dule', data: {host: client.id}});
                        break;
                    }
                }
                break;
            default:
                break;
        }
    }

Looking forward your advice. Thanks

You need to create an onMessage handler on your client side. Refer to this https://docs.colyseus.io/client-room/#onmessage
so lets say your server code is working fine, in the client, you'd need to extract that message.
ex: onMessage(…) {
if (message.action === "request_dule") {…}
}