Group texts in iMessage are a nightmare. If you turn it off in settings, and somebody includes you in a group text, you'll get the initial message from the primary sender - and still get replies from the other unknown recipients.... but you won't know that the original text was to a group, so the second (or third or fourth etc) person replying will appear in your message que as a primary message just to you. It's a disaster.
For example, the group sender could send a pretty picture of a sunset to a group of people, but you've got group texts turned off so you don't know that the message was sent to a lot of other people. Then the next day, one of the unknown recipients to the group replies back saying "I love that!" - and you get a message in your iMessage que just from that person with a message saying "I love that!" And then you're scratching your head wondering WTF are they talking about. Love what?
So then you reply back to that person (whom you may not know) asking - and well you can see it becomes a cluster F%ck pretty quickly.
So don't turn Group Messaging off.
The answer to this hell is to keep Group Messaging On and be a man and Leave the conversation. And after you leave it, send a message to the contact that includes you in a group text to never include you in a group list.
That's all you can do. (And they'll forget and do it anyway). It's horrible (I think).