1) Go to Position
2) Click on Position in which you want to add interviewer
3) Click On Edit button
4) On Detail Page modify position information if you want, Click On Update and Continue.
4) On Hiring Process Page modify hiring process.
Find out step for which you want to add interviewer(Stakeholder)
In Decision makers box, search and select name of user
New name will get added
4) Click on Update And Continue for Hiring Process and Open page
5) Click On Close at Publish page