Email is a good model to emulate, but it's not the system people want for group chats.

If you have to create a new password / answer an authentication check for every server you join that's a ton more friction than discord already.

Yeah but that's exactly what email doesn't do, right? Create account, send email to anyone on any server is the use case it was invented to solve. There are tons of issues with frictionless here if you don't want spam, obviously.