From your event’s guest list, tap the guest’s name that you want to view/add a comment for. The guest’s contact card will appear. Tap “Comments”.
To add a comment, tap "Add new" in the bottom tray. Write the comment and tap the arrow button.
The comment will display in the Comments section.

There will also be a small number notification on the guest's name.

Tapping the green Check-in button will show the Comments screen first. Here you can confirm the comment details and inform the attendee that they will receive an item.
Tap Next to check-in the attendee
