Former members of the doo-wop singing group Sha Na Na were reuniting this weekend for a special one-time-only performance. Sha Na Na is getting back together four decades after performing as the penultimate act at the Woodstock festival (having been invited by Jimi Hendrix) and three decades after a hosting a long-running TV variety show and appearing in the movie classic, “Grease.” Three longtime members of Sha Na Na actually still perform more than 50 shows a year, but this is the first time that Leonard and two others, David Garrett, now a New York City businessman, and Elliot Cahn, a California entertainment attorney who once managed Green Day, will be back on stage together.