There is a page called Account_Overview in the Administration package of your app. You need to add it to the navigation for the user who is allowed to administrate the user accounts. From this page you can create or edit accounts. If you need it more customized, you can use it as input for your own user management.

And if you need users to register, you need to build some register page for guest users as well as a process that creates the accounts for those users (manual or automated process)

For registering new users the Forgot Password module in the Marketplace has a Signup flow in there. You could use that as a start and see if it fits the needs. 

More about this can be found in the documentation: here