I implemented one for work. Basically, you have a websocket for each client, and any message received from client X is transmitted to clients Y and Z. There's not much to it at all; the magic happens client-side.

Here it is, it's really simple: https://github.com/andyet/signalmaster

It's what we use for this (I wrote it).

Check out peerjs.com. Provides a cloud server and has an open source signaling server.

Edit: this is a data solution for now though.