In this paper, we first present a hard decision detector (HDD) for the cooperative networks with the decode-and-forward protocol. Compared to the optimum maximum likelihood (ML) estimator, the proposed HDD achieves the same diversity order while having much lower complexity. Based on the HDD, we propose a new scheme to combine the decode-and-forward protocol with the multiuser diversity. It is shown that the new scheme substantially increases the diversity order from approximately K/2 + 1 to K2/2+1, where K is number of relay terminals. Furthermore, an approximate expression of the average symbol error rate (SER) of the new scheme is derived.