Rujul Zaparde, the extremely eager, very young co-founder and CEO of FlightCar--a year-and-a-half-old company that rents individuals' cars to others--met with Airbnb co-founder Brian Chesky in April, hoping for an investment. He got it, but also got an earful of advice.

"Our first conversation with Brian was the most helpful conversation I've maybe ever had," Zaparde told me. Chesky's Airbnb is a model for Zaparde in building FlightCar because both companies are in the "sharing economy" space, and encounter some of the same hurdles in turning commercial enterprises out of stuff people already own--and taking a slice of business from older, established, regulated industries. "They've also run into some of the same problems we have, so I was really open to advice."

As Zaparde, who is 18 years old, tells it, one of the points Chesky made had fast-growth in mind. "He said: 'When you go at it, you have to make sure you're always pretending like you're launching something new every week,'" he said. "Always be innovating."

That piece of advice, in part, inspired Zaparde to expand FlightCar's business model. In an announcement the company is making today, FlightCar will offer not just rentals of cars owners drop off at airports during short trips (they can potentially earn a few bucks rather than pay hefty airport parking fees of, say, $18 a day), but now also month-long car drop-offs. This move could vastly increase FlightCar's inventory of vehicles at the airports where it operates, Boston's Logan International Airport and San Francisco International Airport. Coming soon: Los Angeles International Airport.

"One thing we've learned is we are a more supply-constrained marketplace. It's often sold out a lot of the time," Zaparde says. He hopes to appeal to city-dwelling car owners to give up their vehicles so they can avoid the hassles, and sometimes the extreme cost, of owning a car in a city--namely, parking expenses and finding street parking. Also, owners earn a guaranteed monthly income ($150 to $400, according to car year and model) from renting out their car.

FlightCar is still a very small company, with fewer than 40 employees. But it says it's growing fast, with revenue growing by 40 percent every month. It was founded in February 2012 by three Ivy-league dropouts, Zaparde, Kevin Petrovic, 19, and Shri Ganeshram, 19. The company launched its first airport service in February 2013 in San Francisco.

Zaparde belives the 30-day option could be a new boon to business. "It definitely has a potential. If we have a standard listing, it's a couple days," Zaparde says. "This is a minimum 30 days, so it's not implausible that it could be 50 percent of the business in six months."

Zaparde says while the company is growing, he tries to keep two other pieces of advice from Chesky in mind. The first: Tell 100 people a day about your company.

"We don't do advertising, so most of our referrals are from friends telling friends," Zaparde says. "You can't pay for word of mouth. It has been the largest driver of users for us by far."