Unlike some other book related sites LibraryThing has no option to leave comments on reviews.
Some users would be happy to add such a feature, others are very much opposed. (See excruciatingly long discussion)
This group is a work-around in place of a review comments feature. If you would like to open up your reviews to public commentary and discussion, you're in the right place.
Here's the process:
1) Create a new thread in this group and put the title of the book under review and your username in the subject line (e.g. Anna Karenina - eromsted)
2) As the first message enter, "Comment area for my review of [Book Title]." Try to make sure the touchstone is correct. Submit the message.
3) Note the number of your new thread. It's at the end of the html address up in the address bar.
4) Head over to your review for the book and edit it to include the following html text at the bottom:
<a href="/topic/topic number">To leave a comment, click here.</a>
topic number is the number of your thread from step 2.
5) Save your review. Click on the link to make sure it worked.
It helps to keep your review and this group open in separate tabs.
If desired, you can also paste your review in the first message or add a link back to your review. A link to your review will take the form:
<a href="/review/review number/">To read the review, click here.</a>
The review number can be found by finding your review on either the work page or your reviews page, and clicking on the little chain-link icon at the bottom of the review. It's the number at the end of the html address.
It seems a bit complicated, but try it once and you'll get the idea.