Hi Anna,
You would have to register the invitee, or change their status to No Response and resend the invitation so that they can register themselves.
To register the invitee, go to Invitee Management > Manage > Invitees & Registrants > Hover over the gray arrow next to their name > Register.
To move an invitee to No Reponse, go to Invitee Management > Manage > Invitees & Registrants > Hover over the gray arrow next to their name > Move to No Reponse.
How do I change a Declined invitee to No Response? (
https://support.cvent.com/apex/CommunityArticle?id=000016136" target="_self)
Best,
Julia