It's very easy to register new users using REST in Drupal 8. This functionality was introduced in Drupal 8.3.
Let's say you are building an app and you want to use Drupal as your back-end. The first thing you would need to do is enable all the required modules (/admin/modules).
You will notice I use the REST UI module. By using this module we don't have to edit .yml files to enable resources, instead everything is configurable in the UI.
The next step is to enable the resource to register users (/admin/config/services/rest):
When the user registration resource is enabled we need to make sure anonymous users can create a new account (/admin/people/permissions):
For the sake of this example we make sure visitors can create an account without a verification mail (/admin/config/people/accounts):
Now everything is setup properly, when we throw a POST call to our back-end a new user will be created:
And in our Drupal back-end we will see a user is successfully created: