This can be as simple as attendee email address or ID, first name and/or last name. A CSV file can easily be exported from a spreadsheet or typically SIS systems will have CSV export options.
Manually register attendees into the system one by one when the need arises such as when an attendee data was not available when other attendees were imported.
Upload a CSV file of attendee data to SFTP for automatic import. This can be scheduled such as overnight.
Connect to the API to import and update your attendee data. This could be run daily or once each semester according to your needs.
Integrate with your Canvas LMS to maintain accurate and up to date attendee information. Typically attendee data is refreshed overnight.