In asp.net web api how to use OAuth2 without having to register users?

I'm building an application with asp.net web api using Ember as a frontend framework which is intended to only allow users to login. What I mean is that you're not supposed to be able to register in any way. All users that will be able to use the application have their information(name and password) stored in a DB. What I want to be able to do is allow them to log in using their information from the DB and also give them different ranks (user and manager). I've through a lot of documentation and even asked friends and I heard about OAuth2 so I went to check it out but in all tutorials they talk about registering and then getting tokens... is there someone who can tell me how I can do this or point me to a guide that shows how I'll be able to solve this specific question?