Collective action through farmer groups can be an important strategy for smallholders to remain competitive in rapidly changing markets. Previous research has analysed determinants of farmer participation in groups, equating participation with group membership. However, within groups the commitment of members can vary, as marginal benefits and costs are not the same for all individuals, and opportunities to free‐ride exist. Low participation in collective activities may reduce the ability of groups to provide useful services to its members. This article investigates determinants of member participation intensity, using the example of farmer groups in Kenya. The role of individual and group characteristics is analysed with survey data and regression models. Previous benefits that members received through the group positively influence their intensity of participation in group meetings and collective marketing, suggesting that reciprocity motives play a role. Low participation can mostly be attributed to structural and institutional conditions, such as group size and the timing of payments for collective product sales. More diversified farmers are less likely to sell collectively when group marketing activities only concentrate on one particular commodity. Since smallholders are often highly diversified, the focus of farmer groups should also be broadened. Further policy implications are discussed.