Fission–fusion dynamics derive from spatial adjustments that animals make depending on resource distribution, resulting in splitting and merging of subgroups. New frameworks propose to classify social systems depending on their degree of fission–fusion dynamics, but little has been done to quantify such dynamics. Operationally defining subgroup is a building block in such quantification. We aimed to define subgroup using interindividual distances (IIDs), while examining the relative contribution of social and ecological factors on the distribution of such distances. We employed a modeling approach using location data collected with a handheld GPS unit from single individuals belonging to a long-term study community of wild spider monkeys and determined the minimum distance at which 2 individuals can be considered in different subgroups. Our results strongly support the crucial role of both social and ecological factors in influencing the way individuals position themselves with respect to other group members. Our socioecological model explained the observed interactivity and interseasonal variation in IIDs in a biologically relevant manner. The critical IID to define subgroup fell within the expected range from field observations. This modeling approach can contribute to the understanding of the factors influencing the evolution of social systems.