Here are two simple steps for making a course available for your members to join on their own:
From the Course you'd like to make available, select "Course Settings" in the header and then select "Change access."
Change visibility to "Members" and then press save.
Now this Course Group will appear in your Library for any signed-in member to discover and join all on their own.
Of course, you can also send personal invites to join any group taking course together.