On a rooftop parking lot, with temperatures in the chilly low 50s, a crowd of all ages shimmied and shook, sweated and smiled as DJ Francis played an eclectic mix of dance music. But this wasn't just another wild South Florida party. It was a special Zumba class for charity, led last month by the creator of the global craze, Alberto "Beto" Perez. The charismatic Colombian in cargo pants — who has become a rock star in the fitness world — climbed onto the roof of a Chevy minivan that doubled as a stage. He demonstrated salsa steps, the merengue march and many other Latin-inspired dance moves — all while also cuing the drummer and the bongo player. For an hour, 75 of his adoring fans — and even the minivan — moved to the beat. "Everybody loves it; everybody has fun," Perez said while...