Select who you want to receive the message, write your message, and then send it off. If you want to start a group chat with friends who have an iOS 5 device it works the same way as sending a group SMS text. With iMessage, Apple took all of that pain away.
Sure, there are other options out there for mobile group chat, but wrangling all of your friends onto one client is unnerving and they’ll probably just uninstall the app in two weeks without telling you.
