The Foxit official website currently restricts only registered users from joining an organization. The user you invited has not yet registered for a Foxit account, so you received this prompt.
As a temporary solution, we suggest you ask the invitee to first register for a Foxit account on the Foxit official website account registration page. Then you can add and authorize the user.
Meanwhile, the Foxit development team is working on improving this process. In the future, we will consider allowing the super admin to add new members directly without the invitee having a Foxit account beforehand.