@aimee Unfortunately I do not think there is a better way. Groups are setup as the course stands at that time and there is no feature in Canvas that detects a new student and automatically adds them to groups. This is probably because Canvas would have no way of knowing which group sets the instructor would want to add them to. For example, a student comes 1 week before the semester ends, do you want the student added to a whole semester's worth of work that student missed or do you just want to exempt them???? While this creates extra work for the instructor it does give you more control of what you want the new student to access.
I know this is not the answer you were looking for!
Nick