This paper investigates applications of multiple-input multiple-output (MIMO) in non-orthogonal multiple access (NOMA) for downlink multiuser systems, where the total number of transmitter antennas is equal to or less than receiver antennas in each user. We also consider a user pairing scheme involving 3 users per cluster with non-overlapping frequency band for users with similar channel gains. The integration of beamforming and user pairing along with successive interference cancellation (SIC) can reduce occurred interference and maximize the sum rate in MIMO-NOMA systems. The novelty of this work is to provide a solution for achieving better performance and massive connectivity in NOMA with 3 users in each cluster by increasing the number of transmitter and receiver antennas. The analytical expressions of the MIMO-NOMA transmission scheme are derived in the ergodic sum capacity term. Also, the impact of the number of users in a cluster on the ergodic capacity of the desired NOMA scheme is evaluated. Finally, computer simulation results are performed to show the superiority of MIMO-NOMA over MIMO-OMA and MIMO-(NOMA-OMA).