The biggest grab for Discord is anonymous accounts. You can put in a username (even a conflicting one due to their #number system) and get going straight away.
Sign-up is a metric that your marketing team loves to track, but is a crazy user bottleneck.
Even if you only do it for, e.g. 6 months-1 year (spam concerns), let people chat with no account/sign-up. Let me give people a link that will let them chat with me, with no sign-up.