We study the problem of user clustering and power assignment for a network comprised of cellular users and underlay device-to-device (D2D) users operating under a non-orthogonal multiple access (NOMA) scheme. Our goal is to maximize the sum-rate of the network by jointly optimizing the user clustering and power assignment. Moreover, we also aim to provide interference protection for the cellular users. The formulated optimization problem is a mixed-integer non-convex problem. Thus, the original problem is decomposed into two subproblems. The first subproblem of user clustering is formulated as a matching game with externalities, where this matching game is solved sequentially while the second subproblem pertaining to power assignment is solved using complementary Geometric programming. Finally, an efficient joint iterative algorithm is proposed that can achieve a suboptimal solution for the mix integer non-convex NP-hard problem. Simulation results show that the proposed algorithm can achieve up to 70% and 92% of performance gains in terms of the average sum-rate in comparison with the general NOMA and traditional orthogonal frequency-division multiple access (OFDMA) schemes, respectively. Moreover, our results show that the proposed scheme significantly enhances the network connectivity in terms of the number of admitted users compared with the traditional OFDMA, NOMA, and D2D schemes.