As @ariyap mentioned, you can search for sponsors in the AD.
However, if you just want a short drop-down list of authorized sponsors in the "Sponsor's Email" field, you can predefine them in that field.
You need to switch the user interface in the sponsor_email field to a drop-down list and then enter the email addresses in the Options field in the format "Email Address"|"Display Name".

The guest will then see this drop-down list during self-registration and can select an entry from the list.

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Regards,
Waldemar
ACCX # 1377, ACEP, ACX - Network Security
If you find my answer useful, consider giving kudos and/or mark as solution
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