From the event, click on the Email tab, from there you can create an email that can be sent to everyone, just primary registrants or only those who checked in at the event.
With the “Email checked-in attendees only” option
selected, organizers can send presentation attachments and links during an event or
afterward. Administrators will need to use the Check In feature on the
event Dashboard to record attendance. You can check in attendees manually or check in attendees with a QR code.