Microsoft US Sovereign Cloud Myth Busters - A Global Address List (GAL) Can Span Multiple Tenants

Michael Gendreau, technically it is possible to flip the UserType attribute from Member<->Guest.  However, Microsoft support considers it unsupportable as engineering has not done the exhaustive testing to determine the impact across all of the workloads.  Between Azure and Office 365, it's dozens of services.  If you choose to use the technique, it's highly recommended that you thoroughly test all the user cases in your environment to make sure it doesn't do anything unexpected.  FYI, Azure AD is seriously considering taking up the effort.  You can provide feedback directly to engineering at https://stackoverflow.microsoft.com.