Sending group messages to family and friends is a great way to communicate with everyone without having to retype or copy and paste a message over and over. The iPhone just so happens to be capable of sending and receiving group messages. Even though the phone used in the video is running iOS 5, this feature has been around for a while and is not iOS 5 specific.
Enabling this feature is simple:
Go to Settings > Messages > Group Messaging and turn it on. That's it!
Now, when you send a group message, if the other user has the feature enabled, they will be able to see everyone in the conversation as well as send a message back to everyone.
Make sure to watch the video for a few more tidbits of information regarding group messaging on your iPhone.
If you are maintaining a lot of group conversations, it can be hard to keep track of them all easily. For more information, make sure to check out this great post detailing how to easily find your group conversation.