Go to the My Courses page and launch your course by clicking on the course icon.
Click the Course Management button in the menu on the lefthand side of the page.
Scroll down and click Add Student or Add Instructor, depending on which type of user you'd like to invite to your course.
A window will pop up. You have the option to send a registration link or share a course code. A registration link can be used by an unregistered user. Only a registered user can enter a course code. For instructions on using a course code, read this article.