allowing external users to access application

Hi,

 

We are designing an 'Certification exam' application in ASP.Net core webAPI and angular  as front end, it will allow only the internal employees of the organization to access the app, they login through windows AD. Evaluators of this assessment is external to organization. They could not given AD access for security constraints. so how do we authenticate the external evaluators without adding them in AD. We don t need facebook/ google/micorosft authentication also. Please suggest