The rate splitting multiple access (RSMA) is a novel transmission technique that integrates both multicast and unicast, enabling simultaneous delivery of common and private message to multiple users. To avoid increased complexity with a growing user scale and enhance system capacity, users are transmitted in groups and configured with distinct message-splitting methods. However, existing research formed groups according to the required services of users but ignored the influence of channels. In this article, a sum-rate maximization problem regarding distinct channels and service requirements of users in a downlink RSMA system is formulated. Then, the origin problem is divided into two subproblems: 1) multiuser group problem; 2) resource allocation problem, which are solved with an exchange group algorithm and an iterative-based algorithm, respectively. Finally, an alternating optimization algorithm is established based on the proposed group policy and power allocation algorithm. Simulation results show that the RSMA system performs better than the non-orthogonal multiple access (NOMA) and the space division multiple access (SDMA) system, and the proposed group policy considering channel and service requirements achieves better system capacity than existing methods.