How to login in my asp.net web application using facebook

Question

I have a web application in which i have two options by which user can log in to it-

1. General user (provided by common password),
2. Priority user(having username and password)).

Now what I want to do is suppose a priority user forgets his username and password than he can avail an option of logging with Facebook account username and password. But again a question is there that is when user logs in a web application how does Facebook authenticate that a user is General or Priority ?

Also my company don't allow user(priority) to register using an email.