Adding Employee Number as a login option

How would I go about adding employee number as a login option in addition too username and email address?

Thank you for your guidance.

I think you would use the username field for it and you can start instantly.
You just need to think about verifying it - in a company usually employee number relates to email - you don’t want someone else register as CEO.

One option to solve that is to register users in an automated fashion using the administration api and just send out invitations that validate email address.