Foreign founders need to have legal status to work in the US. According to the MIT International Student Office website (as of March 2016), International students who are graduating may apply for Optional Practical Training (OPT) that provides legal status for the student to work in their area of study for a period of up to 12 months after graduation (if they have not used any of the OPT time prior to graduation).
Students working in STEM fields may apply for a one-time STEM Optional Practical Training Extension (STEM OPT Extension) that adds either 17 months (before May 2016) or 24 months (after May 2016) beyond their original OPT term.
In addition to the OPT/ STEM OPT extension route, there is another program backed by TechStars called the Global EIR program in which affiliated universities hire foreign founders as "entrepreneurs in residence" and provide them with a legal status to work in the US "until they qualify for their own H1B visa".
Note that Trust Center is not equipped to provide legal or immigration advice. Please visit the ISO and talk to your advisor, or engage legal counsel who specializes in immigration issues to understand the latest rules and to interpret how these rules and policies apply to your personal situation.