He's the guy that Kramer recognizes at the airport in "The Airport" episode. This is just after he and George find out that Jerry and Elaine's flight had been canceled.
(this excerpt is not from the same scene described above)
George: Who's Grossbard?
Kramer: When I lived on Third avenue and 18th street 20 years ago, I had
this roommate who was *always* behind in his rent. Then one month,
he asks me to loan him his share of the rent-- 240 bucks! He took
the cash and >pfffft< disappears. Well, I try to find him, I went
to his girlfriend's house, even his family. Uh-uh. I never got the
money back! He screwed me! And that's the guy-- John Grossbard!