When Jessica Scorpio uses the word “overpopulation," chances are she's talking about cars, not people.

“There are a billion cars on the planet today, and if we don’t do something, in 20 years there will be two billion,” says the Getaround co-founder. “And at any minute, 92 percent of those cars are sitting empty – only 8 percent are being used.”

Getaround is a company on a mission to change all that. Like Airbnb, Zaarly, and other peer-to-peer (p2p) marketplaces, Getaround is an example of how collaborative consumption can have a transformative effect on the way we live our lives and share our resources with one another.

If you own a car, Getaround helps you share it via the web or a mobile app and earn money when you’re not using it.  If you don’t own a car, Getaround takes the pain out of renting one from somebody nearby, in the majority of cases for from $3-$15/hour or $15-60/day. (Owners set the rates, and can adjust them whenever they wish, according to supply and demand.)

Scorpio and her two co-founders, Sam Zaid and Elliott Kroo, were graduates of the 2009 inaugural class from Mountainview-based Singularity University, which strives to turn out entrepreneurs who pursue “groundbreaking, disruptive thinking and solutions aimed at solving some of the planet’s most pressing challenges.”

In order to build their idea – Getaround – into a real company, the threesome had to overcome some major practical and legal issues. “We took on three major challenges,” explains Scorpio, “insurance, legislative, and technological.”

It took the team a year to solve the insurance challenge, but they did that by landing a comprehensive policy from Berkshire Hathaway providing liability, collision, property damage, and uninsured motorist protection.

The team also worked to help get AB 1871 passed in California in September 2010, which allows for personal vehicles to be used as part of car-sharing programs all over the state.

On the technology front, the team developed what they call the Getaround CarKit, a nice little black box for the dash of your car that uses GPS, WiFi and keyless remote technology to allow for easy sharing of cars.

Like Airbnb and Zaarly, both of which we’ve profiled here recently, Getaround is free to join. It takes about a minute to sign up, then the company runs a quick driver reference check, and you can rent a car right away.

(In related news, Airbnb announced Monday that it had closed a massive $112 million round in Series B financing.)

As is the case with all of the emerging p2p marketplaces, at Getaround owners and renters can rate each other, so that a reputation-based community is quickly accreting around the service, matching car-owners and renters to each other on the basis of trust and how others have evaluated them in the past.

The company promotes its service as good for the environment: “Each shared car takes 10 cars off the road and reduces personal carbon emissions by over 40%. As a member of Getaround, you are helping to reduce the negative impact that cars have on the environment.”

Scorpio says that the initial response to Getaround, which launched two months ago, has been rapid and positive. “We’re definitely seeing people from all over the Bay Area signing on -- from Morgan Hill to Mill Valley.”

Perhaps even more propitious is evidence of a pent-up demand all across the country from car-owners wishing to rent their cars through the service. “We have hundreds of cars in the Bay Area but we have 4,000 cars already from all over the U.S.,” Scorpio says. “It’s a tough economy and people want to monetize under-utilized resources like cars.”

The size of this consumer-owned fleet is worth paying attention to. After 12 years in the car-sharing business, Zipcar only has 8500 vehicles in its corporate fleet, by contrast.

Scorpio says the average owner is making about $200/month in profit in the early days, after the 40 percent commission Getaround takes to cover the cost of the insurance and to provide customer service.

Although as would be expected, many of the early enthusiasts for the service tend to be younger people, Scorpio says that the car-owners range in age from “19 to 70.” She notes that p2p sharing makes sense to most Millennials: “Gen Y grew with sharing, we're used to it and we are environmentally conscious as well.”

Drivers have to be at least 19 years old with good driving records. Autos eligible to be included have to be 16 years old or younger and have less than 150,000 mileage on their odometers.

Scorpio says she and her co-founders feel good about what they are doing with Getaround. “We are super-committed to this; it’s really needed. I like that we are helping people be more efficient, have more freedom and get more connected with each other, strengthening our communities. We’re putting idle resources to work.”

Outside of the Bay Area, Getaround is available in San Diego, but the company, with angel backing, obviously hopes to scale nationally quickly. There are a few potential competitors out there, so maybe to build their brand quickly they should license a certain Beach Boys song from the Sixties.