Getting users onto courses is a two-step process. Firstly, a user needs to have a user account created in Moodle--the authentication step. Next, they need to be given access to a course through course enrolment. Both of these steps are supported--and can be enhanced--by plugins. Let's start by looking at authentication.
Log in to your development Moodle as an administrator and, from the Site administration menu, slide down to Plugins, then Authentication, and then click on Manage authentication:
The Manage authentication page lists the available authentication plugins (together with general configuration settings further down the page):
Check out the Moodle docs for details of the authentication process at https://docs.moodle.org/dev/Authentication_plugins#Overview_of_Moodle_authentication_process. What is important to realize about the authentication process is that it isn't simply a question of either creating a new user account in Moodle or verifying user credentials...