MIT's undergraduate math association.
Events to attend to meet your fellow majors. They also have a facebook page.
MIT’s Undergraduate Society of Women in Mathematics.
Events you can go to and tons of resources.
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.
MIT's Office of Minority Education. Participate in a programs,
attend an event,
use their list of resources.
They also have a mentor program