The fact is that you can't easily do the last part without a central server.

There might be some public or semi-public servers for this kind of thing available somewhere, or the alternative that I played with a few years ago was to compress and base64 encode the connection information into a string, and allow users to share the link with friends via whatever method they want, and that can then be expanded client side and used to establish the connection. (Or something like qr codes I've also seen used)

Sadly I never finished that project, so I don't really have any code to show you, but in theory it should work okay.