The Multi-users Multiplexing Radio Voice Transmission method [1] is used for reducing the data overhead of Voice over Long Term Evolution (VoLTE) in the Professional Mobile Radio (PMR) context. In the Multi-users Multiplexing Radio Voice Transmission method, voice packets from different users having the same Modulation and Coding Scheme (MCS) can be clustered into one same LTE packet in the downlink transmission for reducing the data overhead caused by the difference between the LTE packet size and the PMR voice payload. However, in order to increase the voice capacity of VoLTE in PMR context both data overhead and control overhead have to be reduced. Therefore, in this paper, we will present a new Group Radio Network Identifier (Group RNTI) method for reducing the control overhead in the multiplexing scheme. In the Group RNTI method, UEs having the same Modulation and Coding Scheme (MCS) will be clustered in one or more groups. Each group is assigned a common Group RNTI. Control information is sent for groups rather than for each UE. The results show that the Group RNTI can reduce the control signaling up to 85% compared to the fully dynamic scheduling (FD) in the standard LTE in case of using Multi-users Multiplexing Radio Voice Transmission method. The use of Group RNTI for the Multi-users Multiplexing Radio Voice Transmission method allows reducing both data overhead and control overhead issues for VoLTE in PMR context. The proposed method ensures an augmentation of voice capacity and a reduction of blocking rate for VoLTE in PMR context