Hi @Marc
Thank you for posting this in Microsoft Q&A.
I understand you want to enable guest login using their alternate email.
Email is not a supported method for MFA, it's only usable for Password reset authentication only. You'll need to choose a different method for two-factor verification.
To Add Google as an identity provider for guest users please refer below document https://learn.microsoft.com/en-us/entra/external-id/google-federation
Sign-in to Microsoft Entra ID with email as an alternate login ID
https://learn.microsoft.com/en-us/entra/identity/authentication/howto-authentication-use-email-signin
Hope this helps. Do let us know if you any further queries.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.
Thanks,
Navya.