First, You can only connect to a chat room if you have the right room and instance.
IF i suppose that the chatroom is not always the same, what I would do is that I would connect your app with an ajax request to a file you update on the backend (could be a json or even a db) and when someone request to join a specific chatroom to have a flag that check if the person is authorized to do so.
the logic for each player would look like:
1. click a button or whatever to join a chat room
2. ajax call to check if the person is autorize to join it
3. send him or not into the chatroom based on the answer the ajax give you
The logic to update the backend information would look like:
1. Player do something specific that make him able to join the chat room
2. flag him inside the app as authorized to join the chat room
3. when he request it, do an ajax to grab the chat room info to connect him to it.
whatever solution you want to use, as you understood, the all authorization procesd happens outside the chatroom and the multiplayer plugin is not involved at all.