How do I add people to an already existing group chat?
There are currently four people in the chat, three of the members are using an iPhone, and one is using a non-iPhone.
I am trying to add another person to the group chat, how do I do this?
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.)