We study a mean field approximation of the M/M/$\infty$ queuing system. This queue is often used to model the number of cellular phone users in a cell. We assume that congestion here has a positive impact on the performance so that the more there are users, the less it is costly to offer a service per cell phone, for example, if a base station broadcasts a film then the cost per customer decreases. We obtain closed-form formulas for the equilibria. We show that the mean-field approximation becomes tight as the workload increases, thus the results obtained for the mean-field model well approximate the discrete one.