Four studies were carried out to examine how identity fusion, self‐ and group efficacy, and collective action are related and what role self‐expansion plays in these relationships. In the pilot study, participants recalled their experience of participating in mass gatherings. The three other studies were conducted during mass gatherings organized for collective purposes: a music concert (Study 1), a bicycle activist event (Study 2), and Equality Days (Study 3). The results showed (a) a significant positive relationship between personal and group identity fusion, self‐expansion, and self‐efficacy (Study 1); (b) a significant mediating effect of self‐expansion on the relationship between personal and group identity fusion and group efficacy (Studies 1 and 2); and (c) a significant mediating effect of self‐ expansion and group efficacy on the relationship between identity fusion and collective action tendency (Studies 2 and 3).