In this paper, we study multi-user multi-input-multi-output (MU-MIMO) wireless system for spectrum sharing over a limited bandwidth, in which transmit covariance matrices of the coexisting users independently updated in an iterative manner for individual rate maximization. This problem was solved in the literature by an advanced decoding method, in which every user opportunistically cancels the co-channel interference from the coexisting users using multiuser detection techniques, hence termed opportunistic multiuser detection (OMD). This paper also studies the signal-to-leakage-plus-noise ratio (SLNR) based precoding for MU-MIMO channels based on OMD. SLNR improves as interference mitigates due to use of precoding in OMD. Simulation results show the achievable throughput gains of precoded OMD over the conventional single-user decoding (SUD) and plain OMD.