This paper considers the multi-antenna multiple access relay channel (MARC), in which multiple users transmit messages to a common destination with the assistance of a relay. In a variety of MARC settings, the dynamic decode and forward (DDF) protocol is very useful due to its outstanding rate performance. However, the lack of good structured codebooks so far hinders practical applications of DDF for MARC. In this work, a new structured MARC code, one-to-one relay-mapper aided multiuser lattice coding (O-MLC), is proposed. It is shown that, in order to approach the rate performance obtained previously by using an unstructured codebook with maximum-likelihood decoding, it is crucial to use a new K-stage coset decoder instead of the one-stage decoder proposed in previous works. However, if O-MLC is decoded with the one-stage decoder only, it can still achieve the optimal DDF diversity-multiplexing gain tradeoff when the signal-to-noise ratio is high. Simulation results show that the proposed coding scheme outperforms existing schemes in terms of outage probabilities.