Not a problem - they probably missed the window with which to register as invitation links expire after 7 days.
To resend the invitation link, click on your ‘Your Organisation’ icon on the blue left-hand Navigation Bar.
Click on ‘Invites’ then, next to the relevant individual click 'Resend'. They should then get a new registration email.
If you'd like to resend an invitation to an unregistered Beamible member, you can under the 'People' tab click the three ellipses next to their name and choosing 'Invite' from the drop down.