Before revision:  The resulting risk sharing inefficiency is even more intense if the agents' participation in the market becomes an endogenous problem. Regarding this issue, we give conditions under which the participation of an extra agent is bene?ficial for all the existed ones. This discussion naturally leads to the problem of sub-group formation in the market, which is addressed for the ?first time in a financial risk sharing literature. A related example under quadratic utility functionals is extensively analyzed .
After revision:  The resulting risk sharing inefficiency . This agents' strategic behavior, when applied to oligopoly markets of exogenously given financial securities, changes the effective market portfolio and implies a price pressure on the traded securities in the CAPM .