In this paper, a spectrum sharing protocol based on overlay system paradigm is considered. The primary user is composed of a transmitter-receiver pair while a transmitter receiver pair and a relay form the secondary user. The distances between transmitter-receiver pairs are so large; therefore, a relay is required for a reliable communication. All receivers are closer to the transmitter of the other user compared to its own. The secondary user shares its relay with the primary user to improve its performance; in return, it accesses the licensed spectrum. In the first time slot, both transmitters send their information to relay while receiver of the other user eavedrops this signal. Relay decodes the received signals simultaneously and applies bit wise XOR opreation according to physical-layer network coding technique. In the second time slot, R sends physical-layer network coded signal to both receivers which have the information bits of the other user from the first time slot. Both receivers access their intended information bits by applying XOR operation. Bit error probability for the considered protocol is analytically derived and supported via simulation results. Bit error perfomance of the considered protocol is compared with that of direct link between transmitter receiver pairs.