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 New Student page. 

You can read more about inviting students here.

You can read more here regarding invitations not being received.