I believe this is every show more or less. The guys at STERUNIVERSE are pretty good.
Anyway, after a bit of investigation, Ringo's first appearance was August 01, 2000 or 2001 when Vinny gave him the Beatle quiz.
Paul's first was sometime in October 2001 as part of the 911 concert.
And, of course, prior to all that Sean made a telephone call and May Bang was selling her tapes.
Marry Xmas