In recent years, non-orthogonal multiple access (NOMA) has been considered as a promising technique in 5G systems. The users in a NOMA system are cooperative and communicates at the same time slot. This paper proposes a scheme of user grouping to improve the sum rate, by treating the system model as a cooperative game in partition formation. The scheme of user grouping divides users into different coalitions and time slots are allocated to these coalitions. In this scheme, users are allowed to stay alone and communicate with orthogonal multiple access (MA) techniques. Preference relation and sequence game are employed as two algorithms to resolve the coalitional game in partition formation. The simulation results show that the sum rate can be significantly improved by these two algorithms.