Aggregation equilibria of dilute fluids of spherical classical particles are considered using a formalism derived by Lockett and Wertheim. The present theory is a simple application of the Lockett-Wertheim theory, and uses a set of ring, chain, and star Mayer graphs in the grand canonical ensemble partition function. This analysis is applied to a fluid of hard spheres with a thin exterior square well. Calculated here are the equilibrium constants for n-mer association, the n-mer distribution, and the fluid pressure. The shape of the n-mer aggregates is inferred from their individual structure factors.