Both Reviewers and Reviewees can invite Commenters to a review, which allows users who are not the Reviewee or Reviewer to comment on a Review. A Commenter can only add comments on a review after it has been finalised, and will only be able to add general comments to the whole review.
If you have been added to any reviews as a Commenter, you will see them listed on the 'Commenter' tab under 'My Linked People'.
When the Review Status shows as With Commenter, you can select 'View Review' to add your comments to the review.
When you view the form, the comments section is at the bottom, where you can enter your comment. When all Commenters have added their comments, the review is ready for the Reviewee and Reviewers to complete.