In this paper, we consider a Rayleigh fading multiple-access interference channel where a group of users communicate with a receiver in the presence of multiple Gaussian interferers. We derive the closed-form analytical expressions for the ergodic sum-capacity in case of uncorrelated multiple-input multiple-output (MIMO) channel and spatially uncorrelated orthogonal space time block coded (OSTBC) MIMO channel. As the example of special cases, we also show the ergodic sum-capacity for the multiple-input single-output (MISO) and single-input single-output (SISO) channels. Moreover, we derive the analytical expression for the complementary cumulative distribution function (CCDF) of sum-capacity in case of OSTBC MIMO channel. Numerical results are presented and verified via Monte-Carlo simulation.