COSMOS is an astronomical survey designed to probe the formation and evolution of galaxies as a function of cosmic time (redshift) and large scale structural environment. The survey covers a 2 square degree equatorial field with imaging by most of the major space-based telescopes (Hubble, Spitzer, GALEX, XMM, Chandra) and a number of large ground based telescopes (Subaru, VLA, ESO-VLT, UKIRT, NOAO, CFHT, and others). Over 2 million galaxies are detected, spanning 75% of the age of the universe.This is a COSMOS X-ray group membership catalog, combining X-ray group properties from Finoguenov et al. (2007) with estimates for masses and radii calibrated from weak lensing (Leauthaud et al. 2010), and member galaxy information (George et al. 2011). Group redshifts have been determined by searching for red sequence overdensities within 500 kpc of the X-ray centers and are refined by using spectroscopic redshifts when available. We use groups with z<1 to ensure good optical identifications and small photoz uncertainties.