I was hesitant about transitioning from iPhoto to Photos, so after initialing converting to Photos, muscle memory caused me to go back to iPhoto, but I encountered your very question. To regain iCloud photo sharing, go to the iPhoto Preferences > iCloud tab and check the box for Photo Sharing. (The My Photo Stream options are also available.) Subsequently, I transferred to Photos for good about a year ago. Hope this helps.OK, I now have iPhoto running just fine under OS X 10.10.6. Except for one problem. I have been using iCloud Photo Sharing to share with relatives. But now the new shares are not showing up in iPhoto. They do show in Photos. . . . So my question is, how do I get iPhoto to now recognize new shares from others?