The 5G cellular networks has brought along with it a plethora of the connected devices which turned the scales from human-centric to device-centric communication leading to more issues in the spectrum reuse. In this context, this article tackles the resource allocation issue within D2D communications underlay uplink in 5G networks. We focus our study on maximizing the overall system throughput and increasing the number of participating D2D pairs while guarantying the required QoS defined by the target SINRs and reducing the power consumption. We conduct our study in a scenario where the cellular users share their variable bandwidths with a large number of D2D pairs, i.e, the multi-sharing process. We first introduce the optimization problem and provide a low complexity algorithm for spectrum reuse and power assignment. Simulation results demonstrate how our proposed solution managed to increase the overall system throughput and it achieved significant results in mitigating the interference and reducing the power as the system performed well even with high level of transmitted power for the D2D pairs.