There are two ways to record who attended an event: an administrator marks attendance directly, or attendees record their own by scanning a QR code. ("Attendance" — who actually came — is separate from "invitees", who were invited.)
Option 1: An admin records attendance
Open the event from the Events calendar, then click Edit. In the Attendances section, tick each person who attended. Only people who were invited appear here; you can expand the role groups and use the search box to find someone quickly. Click Save.
Recorded attendees are also listed in the event's View event panel under Attendances.
Option 2: Attendees check in with a QR code
Open the event and click Display QR code for attendance recording.
The Event QR Code window shows the event name, its date and time, and the QR code. Choose a QR code size (Small, Medium or Large), then either Open in new tab to show it on a screen or kiosk, or Print it to display at the door.
When someone scans the code with their phone, their attendance is recorded automatically and they see a confirmation: "Your attendance at the event [event name] has been recorded."
For a scan to record attendance, the person must:
have an active role at your site, and
be invited to the event, and
scan during the event — from 30 minutes before it starts to 30 minutes after it ends.
If they've already been recorded, scanning again won't create a duplicate.
To pull attendance across events, use Event reports (Admin → Event reports).