We extend the actor–partner interdependence model (APIM), a model originally proposed for the analysis of dyadic data, to the study of groups. We call this extended model the group actor–partner interdependence model or GAPIM. For individual outcomes (e.g., satisfaction with the group), we propose a group composition model with four effects; for group-level outcomes (e.g., group productivity), we propose a model with two effects; and for dyad-level outcomes (e.g., liking of each of the other members of the group), a model with seven effects. For instance, for an individual outcome with gender as the group composition variable the effects are gender of the actor, gender of the other group members, actor similarity in gender to the others in the group, and the others’ similarity in gender. For each of these models, we discuss the ways in which different submodels map onto social-psychological processes. We illustrate the GAPIM with two data sets.