I believe they board as Chefs, since at the security checkpoint at JFK i was right behind the Turkish crew and pilots and chefs for my flight, and the TSA Security Officer didn't want to let the Chefs thru security first since they didn't had the same flight attendant identification but a Turkish Airlines company identification card on them.

Then the head of the security came while all passengers waiting on the very left security checkpoint, and asked the pilot if he was one of them and whether he knows the chefs in person. The pilot pledged, and they let the Chefs board.

So I assume they don't have the credentials as Flight Attendants, however I saw them sitting on Jumper Seats.