Public registration will reduce the time for the administrator to take actions such as registering a new user. Your customers / contractors will be able to register themselves on the system. 

You can enable public registration on the Configuration/User registration/Public registration page. 

In the settings you need to select a group of users to be assigned to the registered users. There are also a number of options for design the registration page.

Registered users receive a notification to the mail that contains the login details. It is also possible to notify the administrator about a new user. 

To protect against spam, reCAPTCHA is provided, which must be enabled on the "Security Settings" page.