If the administrators want only specific emails to have the authority, then the specified domain has to be a different domain for example "@merakiguestsponsor.net" with only a few emails on that domain like username@merakiguestsponsor.net that can be specified by guests to grant access to them.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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