There are many ways that you can add a user to a course.
A. You can simply go to Courses and click on the Users tab and start adding users to the specific course by clicking on the (+) button next to their name (1).
B. And you can follow the same procedure, if you want to add a multiple courses to a specific user by going to Users and move to the Courses tab (2).
C. You can massively add multiple users to different courses with the import option (3).
You can import existing users directly to courses with the following simplified formats:
Usertocourses;course dummy_login;European History