How to Validate New BuddyPress User Registrations by ID Number

I am working on a member section of my main website which will be in a subdomain for the main website. The member section will be a BuddyPress site. I have everything installed, WordPress, plugins, basic pages ready, etc in the member site and I’m ready to start asking people to register.

This member section needs to only be accessible by employees of a certain company. It’s very important that outsiders (non employees) are not allowed to register.

I considered manually approving all registrations but there are over 8,000 employees in the company and I fully expect the vast majority to sign up.

I have an Excel spreadsheet with every employee’s name and unique company ID number. My goal would be that someone attempting to register has to input their company ID into the BuddyPress registration form and if it matches an ID in the spreadsheet, they are allowed to complete registration.

How in the world do I make this happen? I know it’s possible as I’ve registered for similar sites myself and had to supply an ID number or last name, etc.

Any help and/or ideas would be so appreciated.