My wife got her first MMS today on her phone with multiple recipients. As she responded to the sender, she unknowingly responded to the rest of the recipients. Some of which she did not know. That was followed by text messages "who are you?". She blew it off but then laster tonight she received another MMS. Again she replied back to the sender and it went to everyone else, this time including me.
My brother has a BlackBerry and when someone sends him an MMS in which there are multiple recipients, we all get his response. I figured this was the same thing. When I got home, I looked at her phone and saw that the message was labeled as Group MMS. I looked at the message to see if there was a simple way to just respond to the sender or a way to selectively remove people in the response... No Luck. I called my brother-in-law and asked him if he has this issue on his AT&T iPhone. He said no. After doing some digging I found that solution.
By default, Group MMS is enabled. This allows you to do just this, reply to all recipients on an MMS. I disabled it and the problem went away.
Here are the screen shots of the screen I needed to go through to change this. First will need to get to Settings, then click Messages, then slide the Group Messaging toggle to Off