Employees often engage in collective grassroot efforts to bring about gender equity in the workplace. Such coalition-based advocacy is largely driven by women, which has led to debate about whether men’s involvement as allies can help. Integrating literatures on signaling and legitimacy, we propose that the demographic composition of a gender equity advocacy coalition matters: Men-only groups lack coalition legitimacy, or the perception that they are the “right” spokespersons for gender equity issues, whereas women-only groups struggle to convey issue legitimacy, or the perception that gender equity is of strategic importance within business organizations. By contrast, mixed-gender coalitions signal both forms of legitimacy, and are thus uniquely effective. We demonstrate these effects over three studies: managers reporting on advocacy coalitions comprising their colleagues (Study 1), an audio-based study showing that the proposed effects are unique for gender equity issues (Study 2), and a stimulus sampling study involving multiple policy proposals (Study 3). Our findings highlight the different forms of legitimacy that women and men bring to the table when selling gender equity issues. We discuss implications for who should be recruited to gender equity advocacy coalitions, and why.