When members join via a Sign Up Page to access a specific Course, they'll automatically be added to the Course's Access Group once they start the course. 

How Course Access Groups Work

Course access groups are auto-generated with each Course and cannot be deleted. Any member who starts the Course, regardless of which invite link they used to join the community, is added to this Access Group.

Course Access Groups are required so Heartbeat can track user progress as your members go through your lessons.