Cannot add more people to a group chat in iMessage
You cannot add people to a group message if one or more of the people does not have an iPhone. You also cannot add people to an already existing iMessage group chat if they do not have an iPhone. In this case, one of the people that was in the group chat doesn't have an iPhone. (You can tell because it doesn't have a FaceTime option next to the call and message button.)
I'm on iOS 10 Beta 7 (14A5345a) and while in the message if you tap on the "i" icon on top right you'll then have a button "+ Add Contact"