So basically, change $message_to_id to an array of user ids. Do I need to return $message_to_id from the function or is it just enough to change it? Are there 3 parameters on _before_send like there are on after_send?

I presume that once multiple participants have been added, any future replies will go to everyone i.e. if I send the first message to two people, if one of those 2 people respond, I receive it and the other person from the 2 receive it? Or, is it the case that the _before_send hook has to everytime a reply is sent, work out the to list?