Once you’ve created a design review, you can still add additional reviewers. Open the review and click the ‘Add a Reviewer’ button to the right of the Date in the left drawer. Simply select the reviewers you wish to add from the drop-down list.
Once you add a reviewer, they will receive a notification that they have been assigned to the review. After the reviewer has completed the review and clicked ‘Approve’ at the end, a green check mark will appear next to their profile photo in the review panel, as shown above.