We examine a contest in which two groups compete to win larger shares of a private-good rent, and where group size is endogenized by allowing inter-group mobility of group members. We also relax the restriction on the range of parameter values of the sharing rules adopted by Nitzan ('Rent-seeking with non-identical sharing rules', Public Choice 71, 43-50, 1991, and 'Collective rent dissipation', Economic Journal 101, 1522-1534, 1991) and Lee ('Endogenous sharing rules in collective-group rent-seeking', Public Choice 85, 31-44, 1995). We find that the two groups tend to be of equal size, and that the optimal sharing rules place great emphasis on relative outlays. The rent is 'substantially' dissipated in a collective contest with endogenous group sizes and sharing rules.