In this paper, we consider multiway relay networks with massive number of users. In this situation a fixed transmission scheduling is difficult to apply. We propose uncoordinated communications using the concept of coded slotted ALOHA (CSA), where simultaneous transmitted signals are detected using iterative demapping (IDM) algorithm to improve the success rate probability. We allow each user to transmit the information via any random time slots (during the contention period) to the network. We show the bound of the proposed system and confirm an achievable point using practical coding. We also evaluate the bit-error-rate (BER) performance of the proposed technique via computer simulations. The results indicate that even though with the offered traffic of 1.11 packets/slot, reliable communications is achievable. It is also validated that the proposed system works very well even in relatively low signal-to-noise ratio (SNR) environments. Moreover, the packet-loss-rate (PLR) evaluation shows that the proposed technique outperforms the conventional CSA without simultaneous detection algorithm.