Suspensions of motile bacteria such as Bacillus subtilus or E. coli form a dynamical state exhibiting extended spatio-temporal organization at concentrations near the maximum allowed by steric repulsion. The viscous liquid into which locomotive energy of individual microorganisms is transferred also carries interactions that drive the self-organization. The concentration dependence of collective swimming state correlation length is probed here with a novel technique (bacterial crowd control) that herds bacteria into condensed populations of adjustable concentration. For the free-standing thin-film geometry employed, the correlation length varies smoothly and monotonically through the transition from individual to collective behavior. Using insights from these experiments, we develop a specific model incorporating hydrodynamic interactions in thin-film geometries and show by numerical studies that it displays large scale persistently recurring vortices, as actually observed.