As a Google Voice user who gets to experience life without MMS support, I can tell you that MMS is very alive. I've missed several social engagements that people coordinated via group texts, and a couple of sensitive images that had to be resent over email after the sender was perplexed by my silence...
I suggest Whatsapp. Has group texts, also supports sexting via images sent over data (and video!).
I have my primary number through google voice and I just get a sms saying "MMS Received" and then I get an email with a picture attachment. Not sure about group texts though.
This has only ever been true for MMS received from Sprint users. Google Voice & Sprint's current FAQ indicates that MMS no longer works, even on Sprint [1].