I can think of 2 possible PHP ways to do this:
1. If you want to send the message massively, individually, so that in replying, the user is only replying to the sender: Have you tried to list the usernames/IDs into an array, then loop through the array, sending the mass message?
2. If you want to send the message as a "group", meaning that users can all see who is added to the message, and when replying, replies to the group as a whole, you could use a forum type of functionality. Of course, it would be private, and only individuals added to that message could send/receive messages on it.
Please note, I have not started using Mysidia, however, I'm an active PHP Programmer for other sites, and to my knowledge, Mysidia basically uses OOP PHP for it's scripting, therefore, the same concepts should apply.