When did the White House approach Brokaw about the President's Commission on White House Fellowships?

So Brokaw did this interview with Obama on June 5th and he accepted this position with the White House on June 17th. My question is: when was he approached by the WH for this position? With 28 members on the commission, one assumes that the membership of the commission wasn't put together in a week or so. After all, all the members on the commission would have to have agreed to participate before the list would be made public.