Hey Bleudoggy,
It's possible that they need to enable Shared Albums for iCloud Photos on their devices first. We see that you came to us from How to use Shared Albums in Photos on your iPhone, iPad, and Mac - Apple Support which has the best information on how to enable the feature for their devices.
If they don't use iCloud Photos, you may need to turn on Public Website in your album settings:
"To share an album with friends or family who don't use iCloud, open a shared album that you created, go to the People tab, and turn on Public Website. Your photos publish to a website that anyone can see in an up-to-date web browser. "
Let us know if this helps! Take care.