Registration with Email and Password Should be the default for people who do not have (or do not want to use their) Google Account to Auth.

We already have a provision in the person schema for an email and password:

• Create sessions Use Phoenix Sessions #252

https://github.com/dwyl/app-mvp-phoenix/tree/25fd39c58e892cfbbc5692772631bde1382a6f46#schema

Borrow from: https://github.com/nelsonic/rumbl/blob/master/apps/rumbl/lib/rumbl/accounts/user.ex

Note: our implementation of Auth using email and password will draw on the example in the Book.
If you haven't read Programming Phoenix 1.4, the (basically Official but not free) book written by Bruce Tate, Chris McCord (Creator of Phoenix) and José Valim (Creator of Elixir), start now! 📖
Chapter 5: Authenticating Users covers auth from first principals and is essential reading for understanding how to do it the Phoenix way (using plug).
But Don't expect to understand Chapter 5 without reading the previous chapters. As with anything in CS/programming the knowledge builds on previously laid foundations. Without it you'll be "confused".
I would highly recommend that anyone wanting to learn/understand Phoenix read the book cover-to-cover and write out all the examples e.g: https://github.com/nelsonic/rumbl