... Eric Cantor, Mitch McConnell, Lamar Alexander, and two who had their own amnesty plans: Mike Pence and Jon Kyl.
Per Ben Smith, Sarah Palin was invited and hasn't replied yet; Newt Gingrich and Michael Steele were ruled out due to heading a 527 and a party respectively.
UPDATE: The full text of the letter they sent out is below. Other signatories are Cathy McMorris Rodgers, John Carter, Pete...