This tutorial walks you through the process of enabling or disabling user registration on your WordPress blog. When enabled, this setting allows users to register on your WordPress blog on their own.
The “Register” link is not offered if the Administration > Settings > General > Membership: Anyone can register box is not checked.
By default, user registration is disabled in WordPress. If you want to enable user registration in WordPress, go to Settings > General and check the option that says, Anyone, can register
How to Enable or Disable User Registration in WordPress image
Other than this, If a user registers on your WordPress site, the role assigned to him is a Contributor.
User roles in WordPress :
Administrator – Somebody who has access to all the administration features
Editor – Somebody who can publish and manage posts and pages as well as manage other users’ posts, etc.
Author – Somebody who can publish and manage their own posts
Contributor – Somebody who can write and manage their posts but not publish them
Subscriber – Somebody who can only manage their profile
Enabling user registration’s on your WordPress blog allows you to grow your community but it the same time it opens the risk of SPAM user registrations. So In future, if you want to disable the user registration in WordPress, then just un-check that option.