Q: Can non iCloud users upload photos to shared album?

iCloud: iCloud Photo Sharing overview

iCloud Photo Sharing FAQ - Apple Support

LN

Only iCloud users can add to a shared album.  Anyone you invite can view the shared album, if you publish it as a public website, see:

iCloud Photo Sharing FAQ - Apple Support

Can I share with people who don't use iCloud?

Yes. If you enable Public Website in the settings for your shared album, your photos will be published to a website that anyone can view in an up-to-date web browser.

 

Can people comment on my photos and videos?

Yes. Friends and family with an iCloud account and a compatible device can leave comments and like photos from your shared album. You'll be notified automatically when they do, and they'll be notified if anyone comments back. Learn more about comments in shared albums.

Yes.  They do it via a browser.  Create a public album and send its URL to those to want to view and add to:

Are you sure, OT? I can only view a published shared album at the shared link, but cannot find any controls to add to it or comment on it.

You're right.  One has to subscribe to the shared album in order to be able to add photos.  Tried with my wife's Mac but she's on a very old system, 10.7.5, and can share in iCloud.