Verification of users registering

I run a WP site for my work whereby I’ll have 3,000 or so people using it. I’ve installed a “Category Access” (https://www.coppit.org/code/)plugin to make some posts inaccessible to users unless they’ve registered and logged in.

I’d like people to register, but the thing is, I want to check personally myself that every single person registering is a member of staff here. Therefore, I don’t want the user’s registration approved until I myself verify it as being okay, not the user him/herself.

If anyone has any suggestions how I can do this I’d be eternally grateful!

Kind regards,
Croila