To add a late reviewee to a review cycle once it has already been launched:
Step 1: Navigate to the "Admin" panel found in the top navigation bar
Step 2: Click on "Reviews" on the left Administration panel
Step 3: Find the review cycle you'd like to add someone to and click into it
Step 4: Scroll all the way to the bottom and click "Add late reviewee"
Once the reviewee has been added, they will get the following email notification that will invite them into the system to begin their reviews: