login - How to prevent creation of multiple accounts under the same company?

Let's say there's Company ABC and Employee A signed-up for an account under his own email address (e.g. employeeA@companyabc.com). However, let's say Employee B signed-up under the same company but using his own email (e.g. employeeB@companyabc.com or employeeB@gmail.com).

Our system must be able to detect that Employee B cannot create an account anymore because the Company he belongs to is already signed-up. It's one account only per Company.

What fields should we require on the registration form that will prevent this from happening? They may enter the same company name differently also so we cannot rely merely on the company name (e.g. some may refer to their company as Company ABC or Company abc or ABC only.. but they're actually referring to the same corporation).