Group Play makes it easy to form groups. Clicking on "create group" sets up a Wi-Fi access point for all other devices participating. Others can take part by clicking "join group." The device will automatically search for and connect to an existing group through its Wi-Fi access point. Group leaders have the option to set up a password.
Photo sharing is a feature held over from Group Play on the Galaxy S3. You can select one or more photos to share, and you can also draw on the image or make notes. Each person will show up as a different color to avoid confusion.
Group Play allows you to share music, synchronizing the playback so friends can have a silent disco by plugging in their headphones or set up a sort of surround-sound music concert. One phone acts as the left speaker, another the right, and so on and so forth.
Gaming is another feature added to Group Play for the Galaxy S4. Right now, the number of games available are pretty limited. Gun Bros 2 and Asphalt 7 are a couple that have been specifically adapted for the GS4. Samsung plans to release an SDK for other game developers to jump on board with simultaneous, social gameplay.