Reconfigurable intelligent surfaces (RISs), especially the simultaneous transmitting and reflecting RISs (STAR-RISs), can be integrated with multiple-input mutiple output (MIMO) and non-orthogonal multiple access (NOMA) to improve coverage and save power usage. In this letter, we investigate a RIS-MIMO-NOMA system in a certain scenario, with focus on the system's power consumption. To be specific, instead of the traditional clustering-based inter-user NOMA, inter-group NOMA is proposed to reduce total transmit power of RIS-MIMO-NOMA system. In order to solve the power minimization problem, the beamforming and phase shifts optimization are jointly designed. To address the beamforming optimization problem, the MMSE filters are exploited and iterative method is adopted, and in order to optimize phases, sequential rotation (SR) scheme is proposed. The numerical simulation results reveal the connection between the system's parameters and overall transmit power, and show that for RIS-MIMO-NOMA system, the novel inter-group NOMA scheme is better than traditional clustering-based inter-user NOMA scheme in the total power consumption.