There are several methods of adding learners to a course: through the Users page, the course page, the learner's profile page, the API, self-enrollment, and bulk enrollment.
Via the Users List
Click on People, then on Users. Find the user that you want to enroll and click on the User Enrollment button beside their name. You will be able to choose courses using the Courses drop-down menu. Click Update when you're finished.
Via a Course
Click on the Course that you want to add users to. Click on the Users & Groups tab, then click on Enroll Users. Click on the users that you want to add, then hit the Enroll Users button at the bottom of the window.
Via a User's Profile
After clicking on a user in the Users page, you will need to click on the Enrolled Courses tab. From there, you can use the drop-down menu to add the user to courses.
Via the API
This method requires some technical knowledge as it is done through the API (available to Premium and Enterprise clients only). The documentation for adding a learner through the API can be found here.
Please refer to this article to learn how to enable self-enrolling.
Please refer to this article to learn how to enroll users in bulk.