Q: adding a Yahoo Mail user to Family Sharing?

You are correct.

In my case:

1.  He established an Apple ID a long time ago.

2.  When he received the 'Invitation' by me, the Organizer, to join Family Sharing, he was instructed to open it via iCloud or Apple Mail.  He uses Yahoo.

I think I am explaining this properly.  If I am missing something, please feel free to tell me.  I would love to learn.

Andy

Not exactly correct, I believe.

In my case:

1.  He established an Apple ID a long time ago.

2.  When he received the 'Invitation' by me, the Organizer, to join Family Sharing, he was instructed to open it via iCloud or Apple Mail.  He uses Yahoo. I t is as if Yahoo can not direct him to the final destination.  I am stumbling in explaining it accurately, I know.  Please forgive me.

I think I am explaining this properly.  If I am missing something, please feel free to tell me.  I would love to learn.

Andy

for whatever reason it seems to have worked.  Third times the charm.  Thank You! for your help

James:

I was not sure if you received just your own replies, or saw both conversations.  Apparently, it's both.

I was trying to answer each persons questions directly.

To answer yours:  No, Yahoo is the primary email set up on the AppleID.   He has an iCloud email as a secondary.

But, somehow, after 3 tries, I was able to establish the Family Sharing on his devices.  Not sure why the first ones didn't work.. Most likely parental error combined with old age!  Nonetheless, thank you for assisting and hanging in there!

Andy