Guys,
If you are still facing this issue after rebuilding, try this:
1. Close Photos
2. Go to System Preferences, iCloud, Photos and disable “My photo Stream” and “iCloud Photo Sharing”
3. Go to folder /Users/YourUser/Library/Containers/com.apple.cloudphotosd/Data/Library/Applicat ion Support/com.apple.cloudphotosd/services
4. Delete com.apple.photo.icloud.myphotostream and com.apple.photo.icloud.sharedstreams folders
5. Enable “My photo Stream” and “iCloud Photo Sharing” again.
6. Start Photos. After a while, the shared photos will start coming.
Thank you very much, Hang Yong! Great sleuthing..
I had the same problem but just went through the process of switching over to Photos now on El Capitan, and I saw some differences in the folders located in Containers, likely due either to changes with El Cap or Photos v2.
Specifically, neither the com.apple.photo.icloud.myphotostream nor the com.apple.photo.icloud.sharedstreams folders could be found, so throwing caution to the wind and in the mood to delete folders anyways, I took a stab and moved com.apple.cloudphotosd, com.apple.CloudPhotosConfiguration, and com.apple.ICPPhotoStreamLibraryService to the Trash (didn't empty it initially, in case this really messed things up).
Otherwise following Hang Yong's directions, I started up Photos again and... I still could not see my existing shared folders. And then, after a few minutes, they popped up, and then, very slowly, they repopulated with the photos existing in those iCloud Photo Sharing albums.
Hope that helps anyone now going through that process on El Capitan.