The Sharks are a gang terrorizing the town of Onett in EarthBound. It is comprised of Pogo Punks, Skate Punks, and Yes Man Juniors, and are led by Frank. Ness must face off against the Sharks early on in the game of EarthBound. Their usual meetup place seems to be the Arcade, and according to a Yes Man Junior, to join the gang, you have to beat EarthBound. He faces the various members around Onett. Before he faces Frank though, he must fight a Yes Man Junior. Once Ness beats Frank, he'll be able to enter Giant Step, the first sanctuary of the game.
