The main Photo Stream is for sharing new Camera Roll photos between your own devices not to mention a convenient way to transfer them into an iPhoto Library on a Mac. I believe the iCloud Control Panel can add similar functionality to a PC. It is however limited to one Apple ID.
Only the Shared Photo Stream can be shared with other multiple Apple IDs or even other people with a web address. It can not be setup like the main stream to automatically populate with the new photos in Camera Roll. Only the photos you add to it will be shared with the Apple IDs you choose.
As I mentioned above, I believe DropBox will do what you want, I just haven't tested it out yet.