You can create and/or add to your event's Guest List at any time -- even after you've already started to receive guest's RSVPs.
The Guest List function, however, is not retroactive in that it will not be reflective of RSVPs you've already received. In other words, if you add guest(s) to your guest list who have already RSVP'd, they will continue to show as 'Haven't Heard.'
We recommend deciding whether or not to use a Guest List at the start of your RSVP cycle so that the Guest List can help provide a comprehensive view of who you have and haven't heard from -- and can help you restrict RSVPs to just guests on your Guest List, if desired.
Tags: retroactive, later, after, invitations