Guests are not required to register in order to receive or RSVP to Evite invitations. You can simply click the link on the Evite email you were sent and RSVP directly on the event page. You are only required to create an Evite account if you wish to send an invitation.
For our Premium invites, when you are sent an invite preview to your email, you are requested to log in because you are the host, but guests will be able to click on the envelope to view their card.