If you want to make a comment to a specific participant, you can do it through a Private Message. You can click on that person's username (they are all live links). It takes you to their page where it says Messages. There'll be a box to tick for Private Message. Then you have the option to "Write something to _________" and then send the message. If you don't tick the Private Message box, then everyone can see it, your message is not in any particular thread, just in Messages. But I would do a PM.