The default reviewer will be the workflow owner
Click on the pencil to change the default reviewer and/or add more reviewers. You can add as many reviewers as you like. Once one of them acts on the request, it is considered done.
Hit Enter or Return on your keyboard after each email entry.
Once you have added all your reviewers, click Update.
If you want to add Conditional validation reviewers, click the + button (1) to enter the conditional validation criteria (2) and Reviewer(s).
Our blog post here: http://goo.gl/UUh67f explains the process in details
If you want another person to validate the request after a supervisor approves it, tick the 2nd Validation Step box in your sidebar. Enter the email address of the second reviewer here.