In this paper, the spectrum efficiency of multiuser single carrier (SC) transmission is considered. In SC multiuser transmission, multiuser access can be realized by using interference suppression, e.g., SC frequency-domain adaptive antenna array (SC-FDAAA) transceiver, or by orthogonal frequency allocation among users, e.g., SC frequency-division multiple access (SC-FDMA) transceiver. To evaluate the system spectrum efficiency, the signal-to-interference-plus-noise ratio (SINR) at the output of the SC-FDAAA receiver and the SC-FDMA receiver is derived, respectively. The system spectrum efficiency is then evaluated and compared for both transceivers. It is shown that under the assumption of slow transmit power control, the system spectrum efficiency of the SC-FDMA transceiver does not change with the number of users, whereas the spectrum efficiency of the SC-FDAAA transceiver reaches its maximal value by an optimal number of users, which is determined by both the number of receive antennas and the target receive signal-to-noise ratio (SNR). With this observation, an SC-adaptive transceiver is proposed based on unorthogonal frequency allocation and frequency-domain interference suppression to improve the system spectrum efficiency, and the proposed scheme has been shown to be effective by numerical results.