Q: Is there a way to auto share photos from photo stream?

If you are using separate AppleIDs with iCloud, you cannot share your main photo streams, but can create "Shared Photostreams" on each of your iPhones and regularely add the contents of your camera roll to the "Shared" stream. These streams can be imported on your iMac in iPhoto or Aperture, or viewed in the web.

See: iCloud: Using and troubleshooting Shared Photo Streams

and (for iPhoto) Share photo streams

Regards

Léonie