This paper investigates multiuser detection for space-time block coded (STBC) transmission over a flat-fading multiple-input multiple-output multiple-access channel consisting of J co-channel users each with N antennas and a zero-forcing coherent receiver equipping with M receiving antennas. For the cases in which M=J , it was previously claimed that it is impossible to extend this receiver to general scenarios of orthogonal STBC transmission with J>2 and N>2 . This paper provides a theorem allowing this extension to any scenarios satisfying the theorem condition. Describing in more details, we first prove that zero-forcing receiver of M=J antennas can successfully extend to any STBC transmission over MIMO multiple-access systems which provides an Alamouti-like structure for the inner product of equivalent channels between different receive antennas and users. Then, in order to gain more insight, the theorem role on extending zero-forcing receiver to transmission of orthogonal STBC over MIMO multiple-access systems with J>2 and N>2 , and also to other STBC schemes like generalized coordinate interleaved orthogonal design and Quasi-orthogonal STBC is investigated in more details. Finally, the average symbol error rate of considered scenarios are numerically evaluated and compered for different STBC schemes with various numbers of J and N.