Cannot Add Guest Team Member to Private Channel

arrthurr1111  Figured this out for those who need to know like I did.  I was able to add my Gmail account as a Guest to the root Team, but not sub channels, UNTIL I registered my Gmail account with Microsoft, creating a Microsoft account named my Gmail account.  Once this registration was completed, the Online, Mobile App, and Desktop App all were able to find and add my Gmail named Microsoft account to the Channels.

In short, Teams is waiting on the guest user to register, THEN you can add the registered Microsoft account to Channels. Those that appear immediately have likely registered previously for services such as Microsoft Store or other products requiring a Microsoft account.

EUREKA! 😄