In distributed systems it is often useful to ensure that messages sent between processes in a group are received by all group members. This thesis presents Reliable Non-Ordered Multicast Protocol (RNOMP). We argue that it achieves reliable multicast between processes in groups that processes can leave and join arbitrarily. The protocol has been implemented on top of a group membership simulation which allows evaluation of the performance of the protocol while varying packet loss and the frequency at which processes leaves and joins groups. After analyzing how the protocol handles certain situations we conclude that our protocol achieves reliability and performs well within certain parameter values.