There’s no way around having to know at least one initial peer to discover more.
Fully P2P protocols, such as Gnutella or Gnutella2, or the simpler Overnet (made famous by Storm Worm), are based on each client having a start-up list of a few peers. These can come off a web-based automated tracker for example. The client will discover the whole network or portions of it by asking other peers for more addresses, for example when delegating a file search.
If you truly can’t have any kind of a centralized resource, the best you can do is find the first peer through broadcasted messages and ultimately IP address scanning. The first approach is well-meaning but in at least 98% of cases won’t yield any results. The later approach, of course, is abusing the internet, as well as illegal in most countries.
I really would rethink having some kind of a central tracker. It can be something as simple as a PHP script on a webserver (the gnutella network, today, is held up by ten-twenty such scripts, hosted by people who don’t even know each other). And this sure is more lightweight than email (which, due to spam filters at the very least, would not work anyway).