  Despite the central role of self-assembled groups in animal and human
societies, statistical tools to explain their composition are limited. We
introduce a statistical framework for cross-sectional observations of groups
with exclusive membership to illuminate the social and organizational
mechanisms that bring people together. Drawing from stochastic models for
networks and partitions, the proposed framework introduces an exponential
family of distributions for partitions. We derive its main mathematical
properties and suggest strategies to specify and estimate such models. A case
study on hackathon events applies the developed framework to the study of
mechanisms underlying the formation of self-assembled project teams.
