This women's organization (also called Agudas Nashim of Brownsville and East New York) supported the Lubavitch yeshivos in Brooklyn. The Rebbe wrote to its members regularly, mostly for their annual gatherings, in roughly 1956–1967.
Women and Torah education. The Rebbe encouraged the women's ongoing dedication to supporting Torah education through practical help and heartfelt involvement (IK 7144), teaching that true support for Torah institutions requires both material action and spiritual dedication (IK 5138). He held up Miriam and Devorah (IK 7562) and Esther, whose self-sacrifice every Jewish woman can emulate by supporting Torah education (IK 8955), and wrote about Megillas Rus and the special role of Jewish women at the giving of the Torah (IK 6299_star, 6886).
The home. He wrote about women's unique power to inspire holiness at home and beyond (IK 8993), and about bringing G-dliness into the home through the lights women kindle (IK 9507).
Faith in action. Trusting in G-d while acting through natural means brings true success (IK 7680); a pause in holy work is not an end but an opportunity for renewal (IK 8425). He drew lessons for their work from Lag BaOmer (IK 7301), Yud-Beis Tammuz (IK 8852) and the Shemitah year (IK 9155).
Sources: the Rebbe's letters cited above.