How to add stream dynamically with multiple users with socketio

Question

I've got webRTC to work on my express server but I want to be able to add the stream of the user dynamically. I looked up in the simple-peer docs and found this:

var Peer = require('simple-peer') // create peer without waiting for media

var peer1 = new Peer({ initiator: true }) // you don't need streams here
var peer2 = new Peer()

peer1.on('signal', data => {
  peer2.signal(data)
})

peer2.on('signal', data => {
  peer1.signal(data)
})

peer2.on('stream', stream => {
  // got remote video stream, now let's show it in a video tag
  var video = document.querySelector('video')

  if ('srcObject' in video) {
    video.srcObject = stream
  } else {
    video.src = window.URL.createObjectURL(stream) // for older browsers
  }

  video.play()
})

function addMedia (stream) {
  peer1.addStream(stream) // <- add streams to peer dynamically
}

// then, anytime later...
navigator.mediaDevices.getUserMedia({
  video: true,
  audio: true
}).then(addMedia).catch(() => {})

Peer1 sends a stream to Peer2 dynamically, but it's in the same browser. I'm using socket.io so that people are able to join different rooms. I was using this example to get me started: https://github.com/Dirvann/webrtc-video-conference-simple-peer.

If I use the github example above I understand that I'd have to put:

navigator.mediaDevices.getUserMedia(constraints).then(stream => {
    console.log('Received local stream');

    localVideo.srcObject = stream;
    localStream = stream;
}).catch(e => alert(`getusermedia error ${e.name}`))

In a function. Call init(); then call that function later. But in the simple-peer example it called addMedia(stream) but how would peer2 receive the stream arg if it wasn't in the same browser? In the github code 'stream' is never sent via socket.emit.