The T-Mobile Tech people spent most of November trying to solve this issue and it appeared to be fixed on 13 December 2013, but it seems to be broken again. You can read a rundown here of all the people who were having problems with it before.
A quick run down of the way it works on my Nexus 5 (but it is an OS and phone independent problem since it works exactly the same way on an iPhone).
To restate the problem:
- if ALL the numbers are T-Mobile in a group MMS, then it appears as a threaded group message
- if it is a mix of carriers and I am the only T-Mobile user in a group MMS, it works fine and appears as a threaded group message
- if it is a mix of carriers and there is MORE THAN ONE T-Mobile user in a group MMS, then the group MMS will appear on the phone as if sent via MMS to all users, and all responses back from non-T-Mobile users will be threaded together in a group MMS. HOWEVER the T-Mobile users will receive a single MMS from the other T-Mobile users who reply to the message and those messages will not be part of the threaded group message.
Here's hoping they can revert back to the solution that fixed it before.