Spring boot application with spring security: Only allow certain people to register for the application

I have started creating a spring boot app very similar to the one in this tutorial. The app currently allows users to register and login, enabling them to access protected resources. Right now the login and registration pages are available to anyone. I want to change it so that only people who are sent a link via email (that admins will be able to send) are able to register for the application. Basically I need a way to control who is able to register for the app. I cannot find any info on how something like this would be possible. Is this possible? Any suggestions/links/info on how to do this would be appreciated!

If you're using spring views, you have server-side control to check for parameters and such to see if there is a registration code from an email such as http://<mysite>/registration?code=signupjohn. You can then throw an exception if it's invalid or already been used. Spring Boot does allow you to put resources like that out into the open. You'd just have to create the admin functionality to generate registration keys, store them and email them on your own.