To manually register a user, go to Users in the course area.
Under Add Users, enter the e-mail addresses of the respective users, separating them by commas. Under Group, select the role these users will play in the course (e.g. Students). This will determine the permissions said users have for the course and its syllabus. Then press Add.
If the user already has an account in the system they will be added immediately. If not a message will appear and you should either ask them to join Concourse or speak with your administrator.
The Users page will showcase who's enrolled in your course and to what group they belong. You can change the group at any time by modifying the dropdown and pressing Save.
The more comprehensive ways to register users to a course is through SIS or LTI integration. Integration with an SIS means registration can be automatic and regular. In other words when a student registers for a course he or she is soon added to the appropriate course in Concourse. By contrast with LTI integration students are enrolled on-the-fly as they come from their course in the LMS.