Hello @Matthew , you cannot add self-signed up users (the ones wil mail identity) as Azure Co-Administrator. Only guests that were invited (ExternalAzureAD or MicrosoftAccount identity) can be added.
Pleawe keep in mind that Classic resources and classic administrators will be retired on August 31, 2024. Remove unnecessary Co-Administrators and use Azure RBAC for fine-grained access control, which do support self-signed up users.
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