In group-living species, encounters between groups can range from aggressive to affiliative and often involve multiple individuals exchanging behaviours with the opposing group. Although intergroup interactions are a collective action, individuals differ in their incentives to participate. Individuals must assess the potential payoffs of participation, which can be influenced by a combination of the value of the disputed resource, the individual’s fighting ability, and the fighting ability of their opponent. We examined how these factors affect individual participation in intergroup interactions involving 22 social units comprised of 170 fully-habituated, free-living mountain gorillas at Volcanoes National Park, Rwanda. We used 14 years of observations of 538 naturally occurring intergroup interactions, which ranged from aggressive disputes to peaceful mingling between extra-group individuals. Females participated less frequently in aggressive group encounters than males, and there was no evidence that ecological conditions or individual characteristics created significant differences in participation levels during group encounters. Males were less likely to participate when there were a greater number of adult males within his group, suggesting that group encounters in mountain gorillas may be subjected to the collective action problem. Females demonstrated the greatest participation in peaceful interactions, especially when there were more participants in the opposing group, suggesting that females may benefit from social encounters with extra-group gorillas. In a group-living mammal where intergroup interactions are frequent, patterns of individual participation in group encounters are evidently influenced by the variable circumstances of each encounter, which may contribute to our understanding of the complexity of intergroup relations.