In this paper, we consider a multiuser cognitive radio (CR) transmission where the users transmit and receive information with the help of a two-way half duplex cognitive relay node. The problem of power allocation, user assignment, and the transmission mode selection is investigated in underlay transmission scheme such that the secondary and the primary networks share the spectrum simultaneously. The joint solution is proposed to enhance the secondary system's throughput where the relay power allocation, user to base station assignment, and the transmission mode selection are optimized under a unified framework subject to peak power and inference power constraints. Finally, simulations results are provided to evaluate the proposed schemes.