User registration using Drupal 8's REST API has been in flux but appears to be stabilized (for now).
There are many blog posts on using Drupal 8's REST API. Getting a user registered using REST, however, seems to be in flux. This post will get you set up for using the REST API, and in particular, will outline the steps to get a new user registered on a Drupal 8.3.x site.
First, enable the 4 core Web Services modules: HAL, HTTP Basic Authentication, RESTful Web Services (REST), and Serialization.
If using the REST UI module, go to the UI page at /admin/config/services/rest and enable the User registration resource:
At /admin/people/permissions, enable the permission: Access POST on User registration resource
Under the Account Settings config at /admin/config/people/accounts, allow registration by visitors, with or without administrative approval. If admin approval or email verification is required, the account will be created, but it will be blocked until approval/verification.
If email verification is required, don't send the password in the POST request - just remove the entire line with "pass" in it, see below. If the password is included in this case, the site will return a 422 Client Error with the message "A Password cannot be specified. It will be generated on login."
Here are values for the REST body:
Values for the REST headers:
Content-Type is application/hal+json. The X-CSRF-Token value can be found at /rest/session/token. Since we earlier gave anonymous users the permission to use this REST resource, no authentication is needed.
Your REST testing tool should return a header with "HTTP 200 No Error" and a body with all the new user account details in JSON format.