This can be done by attendancy sheets or with pictures. However GDPR makes these kind of registrations often hard to use.
The Blockchainlab Drenthe (BCLD) researched:
1) how to make registration GDPR-proof
2) how it would be possible to show tamper-free proof of attendancy
3) how to verify trust in the attendee, and
4) how to be sure that the attendees listed were really at the event.
The BCLD developed a solution based on blockchain which is decentralized, transparent, immutable and cryptographically safe to share data on a need-to-know basis. How? Check out this video or contact Blockchainlab Drenthe: firstname.lastname@example.org