Try something like this:
function your_function($params){
/* here is the content of $params:
array(
'sender_id' => bp_loggedin_user_id(),
'thread_id' => false, // False for a new message, thread id for a reply to a thread.
'recipients' => array(), // Can be an array of usernames, user_ids or mixed.
'subject' => false,
'content' => false,
'date_sent' => bp_core_current_time(),
'error_type' => 'bool'
)
*/
$your_id = '123'; // get here all users ids that you need
array_push($params['recipients'], $your_id);
return $params;
}
add_filter('bp_after_messages_new_message_parse_args', 'your_function');
Not tested, though.