Everything you send to the people coming: invitations, joining instructions, reminders, the awkward email when something changes, and the follow-up afterwards. High volume, high visibility, and the place where a small error reaches everybody at once.
Invitations, joining instructions and the reminders that decide whether people actually turn up.
Handling change, answering volume, and the follow-up that decides whether they come back.
The audiences who need different information from everyone else, and where getting it wrong is most visible.
Turning all of this into something reusable, so the next event starts from a standard rather than a blank page.