Meraki doesn't record sponsor email addresses in event logs. You also won't find guest identities (email addresses or usernames) in the logs.
Maybe Splash Access is a solution for you.
https://www.splashaccess.com/
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.