Adding users to my site/database

It’s best to use WordPress’s built-in users system, and work off of that. After all, that’s part of the reason for why it’s provided. ??

If you don’t want users of a certain level to access the Dashboard, use this plugin: https://www.ads-software.com/plugins/remove-dashboard-access-for-non-admins/

One can populate the MySQL Database with User data in varying methods including simply writing a proper script to do so. It’s just an array passed to the DB per:
https://codex.www.ads-software.com/Function_Reference/add_user_meta
(It’s more than that extremely basic example, but may help get you moving in the right direction when you need to add many users).
I prefer (as well noted by James), to force a new user to Register for the site, but many well done plugins also add in the ability to include additional user meta such as a combo of Members and Gravity Forms.