In this paper a simplified parallel belief propagation (BP) algorithm is suggested as a suboptimal soft-input soft-output multiuser detector in an iterative multiuser decoding scheme. The iterative decoding scheme itself is based on applying an outer BP algorithm for message passing between the BP multiuser detector and the single-user decoders. The performance of the iterative decoding process is investigated based on parallel scheduling of the outer BP algorithm. In addition, we analyze the large system BER performance of the corresponding multiuser decoding algorithms. By making Gaussian assumptions on the output of the single-user decoders and the BP multiuser detector, we derive expressions which describe approximately the BER performance of the algorithms. Finally, numerical examples are presented, demonstrating the accuracy of this approximation.