UMA
MIT's undergraduate math association. Events to attend to meet your fellow majors. They also have a facebook page.
ICEO
MIT's Institute Community and Equity Office. They host events and have a mailing list you can subscribe to. You can also find a list of resources, including other mentor programs here.
OME
MIT's Office of Minority Education. Participate in a programs, attend an event, use their list of resources. They also have a mentor program called MAP.