Sparse code multiple access (SCMA) is a promising approach for massive machine type communication (mMTC) which is one of the usage scenarios for the fifth-generation mobile communications systems (5G). Although SCMA improves the system capacity in relatively high CNR, the SCMA performance is poorer than orthogonal frequency division multiple access (OFDMA) in low CNR due to non-orthogonal multiple access. In this work, we propose a user clustering method for uplink SCMA/OFDMA to further enhance the system capacity especially in low CNR. In our proposed scheme, users are clustered into two groups according to channel gain, and SCMA and OFDMA are applied to the groups with large channel gain and small channel gain, respectively. The effectiveness of the proposed scheme is demonstrated by comparing it with SCMA and OFDMA under different user distributions.