What is the iPhone group messaging setting in iOS 4?
You'll notice that Apple added a group messaging setting to iPhones starting with iOS 4.0. The option is off by default. To turn on group messaging:
Navigate to Settings -> Messages -> Group Messaging -> ON. Group messaging takes advantage of the MMS (multimedia messaging service) features of the iPhone. In order to turn on group messaging MMS Messaging must also be ON.
You can always send a text or MMS message to multiple recipients WITHOUT group messaging on. However, if group messaging is turned on everyone who receives the text will see the other people you sent it to in a list. This is similar to an email with multiple CC: recipients or a chat room where you can reply to all.
Each person receiving the text must also have an iPhone with MMS and group messaging turned ON to take advantage of this feature.