when u type “new MultiUserChat(…)” it creates a MultiUserChat object in your application; it does not create anything on the server.
You will be able to join the existing conference on the server by specifying the same room name as the one on the server.
Like what JonWright said, you seem to be operating on a different object when you try join the channel. More specifically, you created a startChat object but called joined on the originChat object. Have you tried startChat.join("…")?