Thanks, Claire, for taking the time to reply to my question.
To clarify, I am an iPhone user and know how to create a shared album on iCloud, and I know how to post photos to my and other people's shared albums.
The problem is for Android users. They appear to be able to VIEW photos in the shared album but not POST THEIR OWN photos to someone else's shared album, to which they've been invited to subscribe. I'm trying to help them out. Note that in this particular case, I am not the one who created the shared album, but I've subscribed to that album and I want to help the Android users share their own photos to the shared album.
Note that step 6 under "Set up Shared Albums on your Windows computer" says "You can now create and view shared albums on your PC. [emphasis is mine] You can find your shared albums in the iCloud Shared Albums app."
It does not mention posting photos to other people's shared albums.
I tried to display the iCloud for Windows User Guide, but it timed out. :-(
I imagine that if iCloud for Windows allows Android users to create a shared album, it must let you populate it...? But I'm not sure because I can't test that out. But what about Android users posting albums to someone else's shared album? That's my focus here.
A group of us went on a hiking trip and I'd like everyone to be able to contribute, not just iPhone users....