Social movement organizing is becoming increasingly dependent on communication technologies. How can Computer-Supported Cooperative Work systems support grassroots organizations in facilitating collective action through democratic participation? In this article, we study Science for the People-Atlanta, a social movement organization dedicated to building a grassroots movement around science activism. We used action research, both participating in the organization and studying it. We use the Freirean model of participatory communication theory to unpack the movement-building process of SftP-Atlanta. The Freirean model provides a three-step, iterative process for sustainable movement building: forming a collective identity, moving toward collective action, and engaging in reflexive dialog. We analyze both the perceived strengths and weaknesses of their sociotechnical practices in supporting the organization as they iteratively go through the phases of the Freirean model. Finally, we identify inclusivity, privacy/security, and social translucence as three values desirable for sociotechnical practices of grassroots organizing.