If a student does not accept your invitation within 30 days, the invite expires. In this case, you’ll need to re-invite that student to your class. Here’s how:

In your Classes dashboard, hover over the class. You’ll see an option to Manage Class. Click on it.

You can re-invite all the students in a class that have an expired invite by clicking the Resend Expired Invites at the top of your class list. 

Find the student in the student list, and click on Re-invite next to their name. This will generate a new invitation for them, and they'll have additional 30 days to sign up.