Hey @r.inks 
Thanks for the post and cool idea! We actually have quite a few users that are building apps like this (where not just anyone can sign up) so I might have a solution that’ll fit better.
What most people do is build a page in the app specifically for admins (or even other users) to invite people to the app. This page would have an input for email (and even other info like name, phone, position etc). The workflow to do this would be to ‘Create an account for someone else’ and in that action you can define those other field values if you plan to include them. Once the account is created, you could do one of two things… Either, send an email with a temporary password and the user would be prompted to change their password the first time they log in. Or, you could use our new magic login feature which sends a link that by passes the need for a password altogether.
Both routes would allow you to not even have a signup form on your app which would eliminate the risk of random people signing up and then you don’t have to deal with email validation logic. 
Give it a try and feel free to reach out with any additional questions to Support@Bubble.io