Let's say I send an invitation with a survey as the RSVP form. People not in our system register for the event. Because they are "anonymous," I have to export the responses in order to get a proper RSVP list. But what if details about the event change and I want to send an update to the people who registered? Do I have to re-import the responses as new contacts in a new group, or is there an easier way to contact respondents?