It is very easy to resend registration emails to athletes if for any reason they missed the initial email--or if you chose to not send the registration emails right away.
To do so, navigate to your team roster page. Click "see all" to expand your roster.
From here, toggle from Team Report to Basic Information.
Click "resend" to send the invitation again to your athletes who may have missed it. You can click "invite" if you haven't sent the invitation email to your athletes yet.
You will then see a confirmation that invitations have been sent.