Whomever starts the group picks the password, and it can be as simple or complex as you'd like. Since Group Play uses Wi-Fi Direct to link the phones, you'll be near enough to just tell others what the password is to join the group.
Password control makes it easy for more than one distinct group to share the same space without interfering.
When multiple people join a group, the leader has exclusive control over the sliding dials for the master volume and for each participant. As a node in a larger network, as long as you're part of a group, your speaker volume isn't your own.
This is what gaming looks like when a bunch of people play together using Group Play. You all see the same moves on your Galaxy S4 screen, which makes for a pretty fun little diversion if you and your friends all happen to have the same phone.
The GS4 requirement is a limiting factor, to be sure, but knowing Samsung, there's a good chance it will migrate Group Play capabilities to its cadre of mobile devices, like the next Galaxy Note smartphones and tablets for a start.