Guest won't appear in the guest list
This event was created outside Google Calendar, so we can't change it directly.
If the event has an organizer specified, once a guest that you invited responds, the organizer of the event will be prompted to invite the new guest. New guests will only receive event update once the organizer has added them to the original event.
If the event doesn't have an organizer specified (for example, like many travel confirmation events), the guest will receive an invitation but will not appear in the guest list on your view of the event.