There are many fantastic math organizations out there, dedicated to supporting and promoting certain groups in mathematics. Check them out!

Organizations and Conferences for undergraduates and new graduate students:

OURFA²M² - Online Undergraduate Resource Fair for the Advancement and Alliance of Marginalized Mathematicians (virtual conference held every fall, typically in November, organized primarily by undergraduates for undergraduates)
SUBgroups - online peer groups for first year graduate-students (registration is typically in late August and late October)
The Math Alliance - “Our goal is simple: we want to be sure that every underrepresented or underserved American student with the talent and the ambition has the opportunity to earn a doctoral degree in a quantitative science.”

Mathematics Organizations dedicated to supporting affinity groups:

National Association of Mathematicians (NAM) - “a non-profit professional organization in the mathematical sciences with membership open to all persons interested in the mission and purpose of NAM which are promoting excellence in the mathematical sciences and promoting the mathematical development of all underrepresented minorities.”
Association for Women in Mathematics (AWM) - “The purpose of the Association for Women in Mathematics is to create a community in which women and girls can thrive in their mathematical endeavors and to promote equitable opportunity and gender-inclusivity across the mathematical sciences.”
Spectra: the Association for LGBT Mathematicians - “arose from a need for recognition and community for Gender and Sexual Minority mathematicians, and we hope that this will be a resource for our community.”
SACNAS - “Advancing Chicanos/Hispanics & Native Americans in Science” (not math-specific, but an excellent group!)

There are a number of organizations which highlight the accomplishments of mathematicians from various communities. Check them out!

Mathematically Gifted & Black - “Featuring the Accomplishments of Black Scholars in the Mathematical Sciences”. Posts a list of Honorees every day in February.
LATHISMS (Latinxs and Hispanics in the Mathematical Sciences) - Highlights mathematicians every day during Hispanic Heritage Month.
Indigenous Mathematicians - “This website seeks to put together all the known information about Indigenous Mathematicians.”
Mathematicians of the African Diaspora (MATHAD) - “MATHAD is dedicated to promoting and highlighting the contributions of members of the African diaspora to mathematics, especially contributions to current mathematical research.”
Spectra’s X Account - Honors LGBTQ+ Mathematicians every day during Pride Month (June)

American Mathematical Society (AMS) - “The American Mathematical Society is dedicated to advancing research and connecting the diverse global mathematical community through publications, meetings and conferences, MathSciNet, professional services, advocacy, and awareness programs.”
Mathematical Association of America (MAA) - “The mission of the MAA is to advance the understanding of mathematics and its impact on our world.”
Society for Industrial and Applied Mathematics (SIAM) - “Through publications, research, and community, the mission of SIAM is to build cooperation between mathematics and the worlds of science and technology.”