Collective action expresses group-based identities, formed by supporters seeking to further particular social causes. While the development of groups linked to action necessitates interaction among supporters, little research has examined how these groups form. Utilizing responses of supporters who participated in 1 of 29 action-planning sessions, this research presents an initial attempt to identify the ingredients important to this process. It shows that to the extent that the actions agreed on in the course of group interactions were seen as capable of making a difference (action efficacy), and worthy of public expression (action voice), supporters' group-based identification was enhanced. This in turn increased their willingness to engage in collective action. Practical implications and avenues for future research to understand the mobilization process are discussed.