Since Simmel's early work on forms of association, the processes guiding group composition have commanded considerable attention in structural sociology, but have not led to a general methodology for examining compositional properties. By introducing a structural event approach, this study offers a new technique that is not restricted to analysis of dyads or triads nor post hoc analysis of those structural arrangements that are observed in a given sample. The approach is illustrated using data on 745 organizational founding teams. Structural event analysis separates choice behavior guiding team composition (with respect to ascribed and achieved characteristics of members) from structurally-induced behavior based on contact opportunities. Results suggest that the strong impact of ascriptive homophily may be tempered when functional considerations of group composition are addressed. However, many of the other arrangements that ostensibly pass as 'functional' are in fact induced by opportunity structures.