A colloidal particle trapped in an optical vortex is restricted to move along a ring of light with constant velocity. When several particles are trapped, hydrodynamic interactions affect their dynamics, resulting in a number of possible modes of motion. We observed that modes that include particle pairing seem dominant. Our results indicate a qualitative difference of dynamics between systems with an even or odd number of trapped particles. Treating a pair of particles as one effective particle, we were able to model analytically the transport properties of an even numbered vortex with no fitting parameters. We find that the average particle velocity is minimized for two rotating particle pairs, increasing with pair number. In addition, we see single file diffusion in this driven dissipative system.