Apple's iOS4 has a convenient new feature tucked away in Settings > Messages called Group Messaging (pictured).
I was trying to figure out what it did, since I’ve always been able to send texts to a group. When I turned Group Messaging ON, it switched the title bar in the text message screen from "New Message" to "Group MMS" (pictured below).
But that was all I noticed -- there was no change in the outbound behavior of my group texts. So I called AT&T and Apple and here’s what I learned:
The Group Messaging setting controls whether or not the complete address list is sent to each recipient in the group.
When set to OFF each recipient only sees you and their name in the thread, but when set to ON things change depending on the recipient's settings.
If the recipient has Group Messaging turned ON then all recipients are shown on that person’s phone.
When a recipient of the message responds it is automatically sent to all members of the original thread.
All messages within the group are placed into a single thread, with each message identified with the sender's name.
AT&T confirmed that sending a text to 100 people (via Group Messaging) only counts as one message sent. If I did the same thing using SMS I'd use 100 messages against my balance. It's basically a dirt cheap way to text. Here's a video of how it works.
AT&T noted that none of their other phones are currently capable with Group Messaging. The video highlights the fact the Droid Incredible 2.1 doesn’t do this right now. This looks to be nice benefit for frequent texters looking for less expensive group texting and true group chat built into the OS.