On Thursday, 18 stories above the streets of Manhattan, the rooftop bar of one of the more than a dozen Marriott hotels in Midtown played host to an unusual crowd. Some were Airbnb hosts, others repped the burgeoning homesharing startup scene, most were wannabe rental empire titans—all were members of New York City’s booming short-term rental industry interested in learning how to turn their Airbnb side hustle into a hospitality superbrand.