To register user using core Rest Module you need 5 steps:

Step 1: Download and enable [Rest UI][1] module and using it's configuration (admin/config/services/rest) Enable User Registration

[![Enable User Registration Endpoint][2]][2]

Step 2: Then in account setting configuration you should allow user to create account, and if you want user to be added using user name and password you will need to uncheck require email verification else leave it checked.
[![Allow site users to create account on the site][3]][3]

Step 3: Now you need to allow anonymous user to access user registration (admin/people/permissions/anonymous):

[![allow anonymous user to register using rest][4]][4]

Step 4: Another step to take is to get a xcsrf token from drupal cause you need to use a post method for user creation and post methods are unsafe methods, so drupal protects them using token, to get a XCSRF Token simply call `/session/token` endpoint.

Step 5: Now final step is to ask for user creation:

Method: POST
Endpoint: `user/register?_format=json`
Header: 

    Content-Type: application/json
    Accept: application/json
    X-CSRF-Token: "XCSRF TOKEN PROVIDED BY DRUPAL"

Body:

    {
	  "name": [{"value": "USERNAME"}],
	  "mail": [{"value": "EMAIL"}],
	  "pass":[{"value": "PASSWORD"}]
    }

and that's it.


  [1]: https://www.drupal.org/project/restui
  [2]: https://i.sstatic.net/zA4gj.png
  [3]: https://i.sstatic.net/Adz6L.png
  [4]: https://i.sstatic.net/kr8mk.png