In this paper, a deterministic approximation for the rate region of multiple access channels is provided when both the base station and the users have large numbers of antennas, and when the transmission bandwidth is divided into several independent subbands. An explicit formulation is also given for the transmit covariance matrices, at each frequency, that reach the boundary of the rate region. From the compact expression of these matrices, suboptimal iterative algorithms emerge that allow the multiple access users to derive autonomously the transmit covariance matrices. This comes at the sole expense of a small amount of signalling overhead, which is constant irrespectively of the number of antennas. Simulations confirm the validity of the theoretical derivations and suggest rather good behavior of the suboptimal self-organization algorithms.