MMS messaging? Not sure if that's possible. iPhone can do this because iMessage doesn't use MMS, it uses an online chat server similar to Facebook messenger hence it can work that way. People who use Android for some reason didn't like using Google's version of iMessage, Hangouts. Like iMessage it can contain both SMS and chat messages in one thread, and have "hangouts" or chat rooms like what you want, where you can add people without starting new threads. For some reason, people didn't use it. LOL.