Both Agron and co-creator Ryan Murphy tweeted the news out today.
Kristen Chenoweth, Harry Shum Jr., and Mark Salling will also appear in the March 18th episode. Murphy will pay tribute to the fans by letting them pick 10 performances for a remix, according to The Hollywood Reporter.
Going to be fun…. #glee100
— Dianna Agron (@DiannaAgron) December 5, 2013
The Glee Christmas album is out! So good! https://t.co/wHJTnSSjZr
Dianna is IN for Glee 100th!
— Ryan Murphy (@MrRPMurphy) December 5, 2013