Yes my friends all have Apple iPhones and are using iCloud.
And yes I am attempting to share Albums and not folders.
I created the shared Album in Photos on my Mac, and then added invitees by selecting their emails as stored in Contacts. In Photos, when I select the "people" icon at the top of the window, I see the names of all the people I had invited. All of them have a question mark "?" in front of the their names. It is unclear/ambiguous what the question mark is supposed to indicate. Hey Apple, why do I have to guess???
Does it mean they received the invite but did not accept? Does it mean Photos does not recognize them as valid (even though they are), or does it mean the invitation was never sent for some unspecified reason? If I select the invitees' dropdown and select "Resend Invitation" Photos does not provide any confirmation of whether the invite was actually sent. The user is just left guessing. This is badly designed, Apple! Why doesn't it generate a "Sent Message" record somewhere -- perhaps in my Apple Mail Sent folder? It is impossible to troubleshoot because Photos gives zero information about what is happening.
I hope someone can help. It seems to me this feature (with so much ambiguity about what is causing the problems as explained above) is very poorly designed and I wish Apple would consider a total do-over of this feature.