Multimedia group communication services in mobile networks as a replacement for traditional trunked radio communication have attracted a lot of attention recently. This paper presents results from a newly developed simulation model to study the transfer of multimedia streams in group communication systems within different setups. As the real time ability of such a system has been identified as the key success feature, the focus of the presented studies is placed on the variation of the users' upload data rate, the resulting overlay network structure and it's impact on the overall transfer delay.