You join a program simply by enrolling in any course that is part of that program. You do not have to complete any additional steps.
After you enroll in a course that is part of a program, that program is listed on the Programs page of your dashboard. Click the program to go to a page that shows more details about the program. From this page, you can view your progress through the program, enroll in or upgrade to the verified track in more of the program's courses, and view any certificates that you have earned.
You can take courses in programs for free, unless you want to earn a program certificate. To do so, you must earn a verified certificate in each of the individual courses in the program. After you earn a verified certificate in each course, you automatically receive your program certificate.
For additional information, see How do I take all the courses in a program?