"We think of it as a gym for the brain,” says Kunal Sarkar, CEO of San Francisco-based Lumosity. The company’s online games challenge users in five cognitive-skill areas--speed, memory, attention, flexibility, and problem solving. Getting to 50 million users has meant convincing people of the effectiveness of the games, which are based on the theory of neuroplasticity--the ability of the adult brain to grow new neurons and connections with the proper stimuli.

In 2005, Sarkar teamed up with his college friend Michael Scanlon, a Ph.D. candidate in neuroscience at Stanford University, and David Drescher to found Lumos Labs. After two years of development, they launched Lumosity.com. Basic membership is free, but premium subscription plans start at $79.95 per year. With $23.7 million in revenue last year, Lumosity earned the No. 403 spot on the 2013 Inc. 5000. Here, Sarkar shares some tips on how to build your user base.

Convince Doubters

People have challenged themselves with crossword puzzles, Sudoku, and Rubik’s Cubes for years. Lumosity had to prove to potential customers that its program of consistent brain training was a better way to improve cognition. The company made its case to consumers by stressing the underlying research and letting users judge the results for themselves. “We help users track their performance over time so they can actually see how they’re making progress,” says Sarkar.

Test Your Assumptions

Lumosity’s founders originally assumed that their market was aging baby boomers. But when the team began to test the games on the public and analyze the user data, it found they were appealing to twenty- and thirtysomethings as well. “Understanding who wanted this and why was a big part of getting to the scale we have,” says Sarkar. Research on how people use the games also led Lumosity to revamp and relaunch its mobile app earlier this year.

Make Users Your Marketers

Scientific data is one thing, but testimonials from actual customers go a long way in winning over new users. Lumosity encourages its users to contribute photos and stories of how the product has helped them. A page on the website shares fan letters from customers, including athletes, engineers, firefighters, and stay-at-home moms, as well as an airline pilot from England who credits Lumosity for helping him pass his pilot’s aptitude test.

Go Global

Lumosity is looking beyond the U.S. It has subscribers from more than 180 countries. Many users come from English-speaking nations, such as the United Kingdom and Australia, but Sarkar has plans to reach a broader audience. The company recently launched its German-language site and plans to roll out in several more languages next year. “We don’t want language to be a barrier to using the product,” Sarkar says.