This paper conducts an analysis and investigation of outage performances and ergodic sum capacity for different users within each cluster in downlink non-orthogonal multiple access (NOMA) systems. Analytical expressions for outage performance and ergodic sum capacity are derived and analyzed. The analysis and simulation demonstrate that NOMA systems with more users in one cluster exhibit higher performance with lower average channel gains, while NOMA systems with fewer users in one cluster exhibit higher performance with higher average channel gains. This suggests the need for different user grouping strategies under varying signal-to-noise ratio conditions, and the fixed user grouping schemes for NOMA systems are inadequate to adapt to complex environments and diverse services. Consequently, this paper brings in the mixed user grouping NOMA systems, which offers greater advantages comparing to orthogonal multiple access (OMA) and conventional NOMA. Finally, this study provides pathways for further research on mixed user grouping in downlink NOMA systems.