Unable to share iCloud photos on iPhone

You have control of which photos you share, and which you do not.

• Share photos and videos with an iCloud Link on iCloud.com - Apple Support
• How to use iCloud Shared Photo Library on iPhone or iPad - Apple Support

And an overview of Family Sharing, if that is enabled here:

• Family Sharing - Apple

In particular, with Family Sharing: “If you’re sharing an iCloud+ subscription, everyone’s photos and files remain theirs alone.”

Shared Albums lets you share photos and videos with just the people you choose — and they can add their own photos, videos, and comments. Read this support article for more information --> How to use Shared Albums in Photos on your iPhone, iPad, or Apple Vision Pro - Apple Support