We are having the same issue on my wife's new HTC
Wife's phone has group messaging enabled, mobile data is turned on. I've changed a bunch of settings but she it makes no difference. Now, she is able to send individual text messages just fine. So far, we haven't found anyone to whom she can send group messages but I am on a Sprint MotoX, her phone is on Verizon. Prior to getting the new phone, group text messages worked just fine.
Under text messages and then connection settings I found some fields that are blank. These include MMS prox and MMS port.
Also, I was considering trying a new messaging app like Messages+ but don't want to buy an app until/unless I think that is a likely solution.
Thoughts, ideas, suggestions?
Thanks!