I figured out the answer to my question on this topic. As some of you have noticed, when you received digests, not all posts have a link to their email, so you can privately respond to them.
However, I just 'tested' this, and it works. Now, if you're using your PHONE, I'm not sure this will work, but on my computer it does.
At the bottom of each individual post (in the digest), you have various options, such as 'View/Reply Online | Reply to Group... ' etc. If the person is sending the message from their email program, such as what I'm doing right now, I suspect you would be able to click my email and email me directly. But if the person is NOT, then here is what I've found works.
Click that option at the bottom of the particular message, that says, "View/Reply Online." You'll likely see the various replies to that particular thread. But below each message, you'll see where it says "Reply." When you click that button, you'll be able to type in a reply. HOWEVER, at the bottom of the reply box, you'll see a little box that says "Private." Once you click that box, the little green box at the bottom of the reply box that BEFORE said "Reply to Group and Sender" will change to a blue box and say "Reply to Sender."