Non-orthogonal multiple access (NOMA) is considered a promising technology for improving the spectral efficiency in fifth generation communication systems. In contrast to orthogonal multiple access (OMA), NOMA allows to allocate one frequency channel to multiple users at the same time within the same cell. Basically, this is possible through power-domain superposition coding (SC) multiplexing at transmitter and successive interference cancellation (SIC) at receiver. For this reason, either an optimal power allocation scheme and an optimal user-aggregation policy result to have a key role on NOMA systems, especially in power constrained scenarios like disaster communications. In this paper, a particle swarm optimization (PSO)-based approach for user aggregation in NOMA systems is presented. The efficiency of this approach in finding the optimal aggregation scheme which require the minimum transmission power, maintaining the quality of service (QoS) constraint of each user, is evaluated through simulations, providing comments and remarks about the obtained results.