In this paper we present work in progress on extending multimedia conferencing standards to scalable, mobile multimedia group support based on SIP initiated Source Specific IP Multicast. We propose extensions of SIP for negotiating SSM sessions. SIP protocol specifications and semantics are compatibly extended without adding new SIP methods. We will introduce a multimedia communication software with distributed architecture as implementation reference. The software is built on a scalable video codec adaptive to heterogeneous network capacities.