I'm reading this as another European and I'm wondering why you are using MMS to have what appears to be a pure text conversation when SMS is cheaper. Is the difference between the two recognised in America or is this another pants/trousers situation?

Thinking on implementation if these are temporary groups all it would need is to clone the messaging app and a add a list of numbers and a start and end date/time and filter the standard message database to show anything that matches. This would mean any non-group messages from someone in the group would appear randomly in the middle of the conversation but that's liveable.

Now, is the messaging app open source...