Airbnb, which lets people rent out their homes to travelers, is exploding onto expense reports, said Tim MacDonald, an executive vice president at Concur, which operates a travel and expense system with more than 22 million users globally. "We're seeing Airbnb going from zero two years ago to $1 million this quarter," MacDonald said. "It's still small, less than 1 percent," MacDonald said of the Airbnb market share. "It's the rate of growth that is eye-popping." The biggest business-travel opportunity for companies like Airbnb, MacDonald said, is in cities that don't have enough hotel rooms, especially during a high-demand period such as a major tech conference in San Francisco: "There's a group of us and we want a house in San Francisco during Apple's worldwide developer conference," MacDonald said as an example. Get the full story at NBC News Read also "Airbnb eyes expansion into 'regular travel services'" at Travel Weekly UK and "Airbnb Launches First Global Ad Campaign in Nine Markets" at AdAge