Justin Yan joined Yale's army the week he arrived in New Haven, Connecticut, as a freshman last fall. He proved himself first as a spy, infiltrating enemy lines up and down the Northeast. Then he was promoted to commander. "It was all downhill from there," Yan says. "I was sucked in."

Yan is one of more than 40,000 college students who have become addicted to GoCrossCampus, or GXC -- an online game developed by a group of current and former Yalies. The rules are similar to those of the classic board game Risk: Teams of players from various schools battle for territory on a map that reflects real campuses, right down to landmarks such as dorms and dining halls. More than 2,000 kids from the 12 schools in the Atlantic Coast Conference competed in a recent championship match, and 5,600 participated in an Ivy League battle. ("The single largest competitive event in the history of the Ivy League," GXC's website boasts. "Prepare for awesomeness.")

With the help of a pair of campus organizations that support undergraduate entrepreneurs, four students founded GXStudios in 2007. They are Matthew Brimer, a Yale senior, and 2008 grads Brad Hargreaves, Sean Mehra, and Jeffrey Reitman. (A fifth co-founder, Columbia senior Isaac Silverman, has since left the business.) The company makes money by selling sponsorships to advertisers such as Coca-Cola and Princeton Review. Though the students won't disclose what their revenue was last year, they say GXStudios is profitable.

The founders of GXStudios believe they are pioneering a new kind of interactive formula, something they call team-based social gaming. The concept combines a handful of variables that exist in isolation online and in the real world but have not been combined as they are in GXC. Users speak with one another through a chat function, and the interactions are goal driven rather than purely social. Think MySpace meets World of Warcraft, with a heavy dose of teamwork. "Take a look at Facebook or any pure social networking site, and there are all sorts of ways to interact, but there's no inherent motivation to do so," says Brimer, a sleep-deprived sociology major who serves as GXStudios's chief marketing officer.

Although the game is no longer limited to students and now offers tournaments based on seemingly arbitrary preferences such as Star Wars characters and favorite brands of soda, it began on university campuses, and its success relies on those age-old rivalries. "The general idea is that people have real pride in certain of their affinities, obviously starting on the college campuses," says Dan Googel, a principal at Easton Capital, a venture capital firm that has invested more than $1 million in GXStudios. "Not everyone can be the quarterback on the football team, but this was a way to engage the rest of the campus to participate."

The funding came about in an interesting way: Mehra and Reitman first contacted Easton Capital when they were in high school in an effort to secure financing for an artificial bladder they had developed for an invention contest sponsored by a large pharmaceutical company. Easton passed on that idea before agreeing to back GXStudios.

In a bid to expand beyond the academic world, GXStudios has created a second gaming platform -- a team-building tool for companies. Google, Vivendi, and others have already run the game, pitting employees in one city against those in another or the sixth floor of an office building against the seventh. And this month, GXStudios plans to set up a new site, PickTeams.com, to host a series of team-based social games that move beyond the Risk playbook.

The start-up now has a Manhattan office with about a dozen employees. The popularity of the company's games has thrilled but not necessarily surprised a group of founders convinced of the power of its idea. Still, running a venture-backed start-up is a heady experience. "It's one of those things I never would've predicted when I came to Yale," Brimer says. "If someone would've told me that, when I graduate, I'd go to work for the start-up company I founded and am working for already part time, I'm not sure I would've believed it."