How to turn on Group chat on iPhone 4S?

I want to talk to 2 of my friends through group chat, how does it work and how do you do it?

If you mean standard iphone messaging, you just got back to the messages screen (where you can see all your conversations with other people) tap the square with a pen button on the top right, enter both of their names in the top box, then type what you want in the bottom box (above the keyboard), hit send. They will both get your message and whatever one send back, you and the other friend will both get