There are a couple of ways to add comments. One way is to use the Disqus commenting system at the bottom of each page. This might be helpful for when you want to add semi-permanent commentary to a page or provide material for others to discuss.
The other way is to use the Hypothesis system for social reading with collaborative annotation. This is especially helpful for me if you want to comment on the use of a particular word or want to point out errors related to grammar, spelling, or clarity.
In the upper-right hand corner of each page of HIPAA from Scratch, you'll see a leftward-pointing arrow that will open the Hypothesis sidebar.
Click to open. You will see a link to Sign up / Log in: You'll need to register to add comments and annotate.
Once you're logged in, you can not only add a page note (for the entire page) from the sidebar, but you can also highlight a word or a passage, and then add an annotation for just that text. After you add it, it will be sent to me.
For a detailed guide on Hypothesis, see Annotation Basics at the Hypothesis site.