You're right it is mms. But Apple needs to find a way for it to change just as it changes for individual texts. Individual texts go from imessage to sms. They should be able to change it from imessage to mms in group threads. I've seen when placing just 1 non apple number in a new group thread, the entire thread is in green (mms). When it is all iPhones the entire thread is blue (imessage). It should be that when one of those number goes android, windows, or turns imessage off, the entire thread switches to mms. But that is not what's happening. It stays on imessage thus you not receiving the texts.