Email to the database? You have to add the users to WordPress and not in the database.
If you want a plugin more user-friendly interface for user registration and/or Front-end Login and/or password reset. The next plugin is very easy to use it:
https://ww.wp.xz.cn/plugins/ppress/
Not to the actual database. I want to add them in the backend in the plugin.
What I want to do is, I have a list of email addresses that I have from previous customers. And I want a section of the site protected and the only way they can gain access is by inserting their email. So their email would serve as a password basically.
This is more complex to program, as well as unsafe for users and for the health of your WordPress.
I do not think there is a plugin for this