It's too bad Google Nest does not note the Google account requirement in its "Invite members to a home" documentation. All it says is "Enter the name or email address of the person you want to invite to your home." And when the non-gmail account invite fails, there's no indication as to what the problem is.
Due to inactivity, I'll go ahead and lock this thread in 24 hours. Feel free to start a new thread if you need assistance with your Google speakers and we'd be glad to assist you further.