Here's a bookmarklet I threw together. You can go to the link given by someone else and then click the bookmarklet to join their session. It messes up if you change pages, though. Maybe there's a way to fix that.

Pretty interesting that it maps reactive UI elements like bootstrap divs which resize properly!

<a href="javascript:( function(){ var%20s=document.createElement('script'); s.setAttribute('src','https://togetherjs.com/togetherjs-min.js'); document.getElementsByTagName('body')[0].appendChild(s); setTimeout(function(){ TogetherJS(this); }, 100); })();">LINK</a>