Hi there, many of us T-Mobile users are having this issue.
Please refer to this thread:
Group messaging problems | T-Mobile Support ... view
post 24 as well. I have the same username on there.
The original thread about this issue was actually removed or blocked on T-Mobile's support site I assume was because I posted T-Mobile's Executive Response email addresses. They should be easily found by doing a Google search, and I recommend anyone with this issue reach out to them, provide your phone number and account holder information.
They have been in touch with me. At first they wanted a highly trained technician to reach out to me, however I don't know what that would solve. This is not a software issue since this is affecting all T-Mobile devices...Android, iOS, and Windows. After some back and forth emails, I was told to hang tight because they were receiving a ton of similar complaints. Then they recommended I use third party software to remedy the situation, but while Group MMS works with it (Google Hangouts), I cannot send out messages via WiFi Calling which is a must for me, as well as the interface being terrible...plus, Google Hangouts isn't available on iOS or Windows.
The more people who reach out to them, the more likely it is they will do something about this. Again, please refer to the thread above.
Thanks!