To remind reviewers of your direct reports to complete their reviews, go into the review cycle from the "Your Team" tab by clicking on the "Reviews" tab on the left.
From here, click on "Show reviewers" next to your direct report's name. You will see all the reviewers listed and whether or not they have completed their reviews.
To remind them, click on the direct report's name and select the "Remind" button
Reviewers will receive this email notification: