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Good Morning All,
I have solved the problem. In case someone else has similar, here's what I did.
I eventually remembered why I had my outlook.com account. (FYI, MS set it up so that Outlook (with my family MS Office subscription) would work on my iPhone without ads). However, that is no longer needed. I set my home email as primary and deleted the outlook.com email. I was then able to accept the invitation.
Seems pretty unique, which may be why no one responded. But you never know. "the only reason I'm singin' you the song now is 'cause you may know Somebody in a similar situation / Or you may be in a similar situation, and if you're in a situation like that," then this is one thing you can do.
Good luck!