I have seen this problem mentioned by several other users but it seems the issue has never been solved or adequate support provided. I have spent countless hours on the phone with customer service / tech support and the "executive relations" team...although they struggle to call me and instead will only e-mail which takes weeks on each iteration of our back and forth.
I am not able to receive MMS from T-Mobile users in a group chat. I receive all the other messages but when the T-Mobile user responds, the message goes to everyone other than me. I have went through all the "simple" steps...there must be something wrong on the back-end with Verizon's services and/or the porting of my phone number initially.
Trying another route by posting here to get some help as everything else has been unsuccessful at this point and I am getting extremely frustrated.
I am very sorry for the experience as I know the importance of staying connected with everyone. I s that same user able to send you an MMS directly? What model is the phone that you have ?