Device-to-device (D2D) communications is an emerging service model that is currently under standardization by 3GPP. Although D2D offloading has a great potential to relieve increasingly congested cellular networks, its benefits come at a cost, namely interference. Most of the prevailing D2D designs conservatively avoid interference via either spectrum resource allocation or power control. These works however do not exploit the spatial degrees of freedom (DoF) inherently supported by a multi-antenna device. In this work, we present MD2D, a multiuser D2D system that embraces concurrent D2D transmissions, while leveraging MIMO techniques to actively eliminate interference across D2D pairs. MD2D has a systematic methodology that checks whether the antenna combination in a D2D network is capable of eliminating cross-pair interference, and thus ensuring interference-free concurrent transmissions. If the interference can be eliminated, then MD2D applies a bucket-based DoF assignment algorithm to determine an effective antenna usage configuration that handles the interference. We evaluate our design via testbed experiments and simulations. The results show that, as compared to the traditional interference avoidance scheme, MD2D improves the throughput by 87.39% and 236.02% in a three-pair testbed and in large-scale simulations, respectively.