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We present a user-scheduling scheme for multiuser multiple-input-multiple-output (MIMO) systems with random unitary beamforming (RUB) in this paper. The new scheme, which is termed as adaptive beam activation based on the conditional best beam index feedback (ABA-CBBI), requires low average feedback load by imposing a feedback threshold on the users' signal-to-interference-plus-noise ratio (SINR) and suffers less multiuser interference by only activating those beams requested by at least one user. We derive the exact analytical expression for the sum-rate capacity of the resulting multiuser MIMO systems, based on which we examine the optimal selection of the feedback threshold in terms of sum-rate maximization. We demonstrate through selected numerical examples that the proposed ABA-CBBI scheme with optimal thresholds can achieve better sum-rate performance than existing schemes over high-signal-to-noise-ratio (SNR) regions.