I am new to this site. I asked a question yesterday and had some good helpful feedback - how do I reply to these people?

To the right and below the post is a message that says "reply to". You can reply right there on the post.
If you wish to send someone a private message, then click on their name on the post which will take you to their profiles. There is a place that says "message" below their avatar picture, if they accept private messages. You can click on that and send a private message or a public message to their profile.

Another way to communicate is to click on the persons name and that will take you to their profile page and you can send a message. It can be an open message or a private one. Check your profile page now...

To make comments to each person individually you just need to click/tap the "reply to (whoever)" below their comment. But you can post a general reply to everyone by using the "post answer" bubble, in many ways that is better than replying to each person individually because it is more apt to be seen by everyone.