When Karl Groeger Jr. wanted to round up people to meet a local band at the record store he recently took over from his dad, he spent several days passing flyers around town.

That was in 1991. Nowadays, Groeger posts on Instagram that a Long Island legend, a Jonas Brother or another high-profile celebrity will visit Looney Tunes Record Store, and within hours or even minutes, fans gobble up hundreds of meet-and-greet tickets.

Since March, Cardi B , Mariah Carey , Joe Jonas , Migos rapper Offset, and life and musical partners Selena Gomez and Benny Blanco have all popped into the West Babylon music store to interact with their fans.

“People ask me this all the time, ‘How is it possible?’ ” Groeger told Newsday one recent afternoon. “It's just kind of what we do.”

Over the past t

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