In this video, we go over what a Business to Business (B2) external Azure AD account is and how to enforce Multi-Factor Authentication (MFA) on guest accounts with a Condition Access Policy.  The demo goes over disabling SMS authentication, then inviting guest users to the tenant.  After that, we walk through enabling MFA with the Microsoft Authenticator app.

Links:
Zero to Hero: https://www.ciraltos.com/zero-to-hero-with-windows-virtual-desktop/
Monthly Active User Pricing: https://docs.microsoft.com/en-us/azure/active-directory/external-identities/external-identities-pricing