This study investigates the opportunistic joint source-pair and relay selection policy for cooperative networks. The investigation considers multi-user pair and multiple bi-directional amplify-and-forward based relays in the system model. The strategy selects an appropriate user-pair and relay amongNcandidate user-pairs andRavailable relays based on the maximum achievable sum-rate of each user pair and related relay terminal. Analytical, asymptotic, and Monte Carlo based computer simulation results reveal that the joint-selection strategy achieves multi-user and cooperative diversity order in high signal-to-noise ratios. The results also show that the opportunistic joint source-pair and relay selection strategy outperforms the max-min based selection policy. Moreover, the system contains a single relay, the joint-selection strategy achieves only a multi-user diversity and performs worse than the system that contains multiple relays. The results also show that the total achievable diversity order equals to the total number of possible paths between user-pairs and relays, which is$L=N\times R$L=NxR, while in the multi-user and a single relay model, the diversity order only equals toN. Results also reveal that the proposed selection strategy provides an equal transmission opportunity for each of the user-pairs.