Unmanned aerial vehicles (UAVs) are regarded as powerful assistance for emergency communications due to their disregard for the limitations of the geographic environment. In this paper, we consider a multi-UAV-assisted wireless emergency communication system, where UAVs are applied as aerial base stations to serve terrestrial device-to-device users (DUs). Our goal is to maximize the UAVs' energy efficiency (EE) through the user grouping strategy with a joint optimization scheme regarding UAVs' trajectories and transmit power. To deal with the resultant mix-integer non-linear programming problem, we divide the optimization process into two stages. In the first stage, we discretize the trajectory into a set of stop points (SPs). Then, the grouping of DUs is achieved by pre-planning the location and optimization range of SPs. In the second stage, with the determined DU grouping strategy, we apply Dinkelbach method and successive convex approximation to convert the original problem into a solvable convex optimization problem. Finally, simulation results verify the effectiveness of our proposed algorithm, which has better performance compared with benchmark schemes in the low user-density region.