Nextjs authentication

Question

I'm doing the first step into Nextjs and I'm stacked in defining the authentication part. I understand that Nextjs tipically rely on NextAuth for authentication. My first thogth was to enable provider like google but I don't understand how to avoid new user registration. The application I'm thinking need to have a login to protect routes but not the registration part. Basically I need to decide on my own who can access.

In realty I started working on an express backend that rely on session cookie and MongoDb (no jwt involved) but now I'm a bit confused on how to proceed.

Is there a way to avoid registering new user with providers? What is your suggestions?

Many thanks