If you want to share photos with other Galaxy S III owners and let them keep the files, you have to use a different feature called Share Shot, which automatically distributes the photos you're taking by sending them over Wi-Fi to other S III handsets on your local Wi-Fi network.
Group Cast: With Group Cast on the Galaxy S III and a Wi-Fi network, share and collaborate on documents, presentations or images in real-time with multiple friends or colleagues without loading the file separately.