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System capacity is considered for a group of interfering users employing single-user detection and multiple transmit and receive antennas for flat Rayleigh fading channels with independent fading coefficients for each path. The focus is on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. It is shown that the optimum signaling is sometimes different from cases where the users do not interfere with each other. In particular, the optimum signaling sometimes put all power into a single transmitting antenna, rather than divide power equally between the different antennas. We show that either the optimum interference-free approach, which puts equal power into each antenna, or the approach that puts all power into a single antenna are optimum for the majority of signal and interference powers and we show how to find the regions where each approach is best.