In the Pick-up Management (which is available in the group registration after tickets were distributed) a new template can be sent to the group visitors.
This new invitation template can be uploaded in Group Registration - Group Setup - Templates - Invite GroupVisitor. The same replacer and mergefields can be used as the existing "Ticket pick-up document" template.

In addition it is now possible to send the invitation or the voucher (for self pickup) to the whole group. Group visitors which already received an invitation or the voucher won´t get the e-mail a second time.