If your student did not set up their own account, but you have already created a profile for them in your My Students area, then you can click on that student's name and underneath you will see the option to " Resend Invitation" (see image below). 

If you have not yet created a student profile for them, and want to invite them again, you can send an email with the registration link from your Add a Student page. You can also create a profile for them by clicking Add a Student in My Students, and then entering their name and class and going to their profile page to invite them using their email. 

You can read more about inviting students here.

You can read more here regarding invitations not being received.