How do social movements broaden their societal reach and mobilize more participants? To date, research has focused on social networks, organizational coalitions, and computer-mediated communication to aid in mobilization, but most research assesses these mechanisms in relative isolation. This paper integrates these perspectives to explore the ways social movements expand their ranks through large-scale street demonstrations and marches. Analyzing data collected through random surveys of participants at two of the largest demonstrations in the history of the climate movement-the Copenhagen Climate March in 2009 and the People's Climate March in 2014-we assess how the movement expanded. Consistent with the literature on protest mobilization, we show that the climate movement increased participation at these events via personal networks, organizational coalitions, and computer-mediated channels of communication. In addition, we find clear evidence that large-scale protest events like the People's Climate March are bringing new people, including passive members of social movement organizations and disengaged sympathizers, to the streets and into the movement. The paper concludes by discussing the implications of our findings on social movement expansion through large-scale demonstrations and outlining opportunities for future study.