You can turn iCloud Photo Sharing on and off independently of My Photo Stream.
Choose iPhoto > Preferences, and click iCloud.
Do one of the following:
To turn on iCloud Photo Sharing: Select the Photo Sharing checkbox.
To turn off iCloud Photo Sharing: Deselect the Photo Sharing checkbox.
After you turn off iCloud Photo Sharing in iPhoto, you can continue to add items to and remove items from your shared photo streams using your other Apple devices.