And if the receptionist cannot find a course for which they can register, you are left with a Courses History table that is linked to the contact but not to any course. I would suggest not creating a new record in the join table until the user clicks a control to register the contact for the course.
You might find the "check boxes" layout in this many to many demo file a source of ideas for what you want to do: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
If you are using FileMaker 12 or newer, open this file from the File menu to get a copy converted to the newer file format. The method shown can easily be enhanced in a number of ways to better facilitate the process of searching out courses for which to register the contact.