The physical layer authentication (PLA) in the fast-varying uplink channels of massive multiple-input multiple-output (MIMO) system is considered in this paper, and a two-user cooperative PLA scheme is proposed. In the proposed PLA scheme, the messages and embedded tags of users are transmitted in a space-time modulation model with the using of unique decomposition constellations. As a result, non-coherent signal detection is carried out at the base-station (BS). The approximate symbol error rates of message and tag for the proposed scheme are given as well as the probability of false alarm and detection probability. Simulation results show that the proposed scheme achieves high reliability with low latency while the number of BS antennas is large, and meanwhile, the cooperative authentication performance is better than that of independent scheme.