For adding a reviewer to a Shared Review or an Email review, please follow the below mentioned steps:
1) Launch Acrobat and open the Comment pane.
2) Click "Track Reviews" link listed under the review accordian.
3) The Tracker window will open.
4) In this window on the left hand side, select the review in which you want to add more reviewers.
5) On the right hand side there will be a link "Add Reviewers". Click this link and proceed with the workflow to add new reviewers to the review.
Do let me know if you face any issues.