There is increasing interest in understanding the processes that maintain phenotypic variation in groups, populations, or communities. Recent studies have investigated how the phenotypic composition of groups or aggregations (e.g., its average phenotype or phenotypic variance) affects ecological and social processes, and how multi-level selection can drive phenotypic covariance among interacting individuals. However, we argue that these questions are rarely studied together. We present a unified framework to address this gap, and discuss how group phenotypic composition (GPC) can impact on processes ranging from individual fitness to population demography. By emphasising the breadth of topics affected, we hope to motivate more integrated empirical studies of the ecological and evolutionary implications of GPC. Members of animal groups often vary in their phenotypes (e.g., personality, morphology). Many recent studies have shown how different aspects (e.g., phenotypic average, variation, extreme) of GPC affect group-level outcomes (e.g., foraging success, mating system). Group-level outcomes can shape selection when individual and group phenotypes co-vary. Selection arising from GPC can drive changes in traits that affect aspects of individual-to-group covariance, such as behaviours that determine group membership or by changing the expression of traits (via phenotypic plasticity or indirect genetic effects). A framework based on interacting phenotype theory can quantify the selective consequences and evaluate the evolutionary implications of GPC. This framework is still largely unexplored empirically but it is applicable to many topics in evolutionary biology.