I have an iPhone 6 running ios 8. When group messaging, I noticed that sometimes it may not work as expected. Lets suppose I made a new group text. Some messages would show up in the group text thread while others would show up in a separate thread. Sam makes a group chat with Carly and Jessi. Carly replies back and it appears in the same group text conversation that Sam had started. When Jessi replies back, it shows as if Jessi is only messaging Sam (aka separate thread).