Supporting Quality-of-Service resource reservations for IP multicast flows is especially advantageous for distributed multimedia applications like video conferencing, 3D tele-immersion, or multi-player online gaming. In response to various limitations of RSVP the IETF developed more flexible signaling protocols within the Next Steps in Signaling (NSIS) framework. But unlike RSVP, the NSIS protocols were designed to consider unicast flows only in order to reduce protocol complexity. This paper presents an extension of the NSIS signaling protocols that allow for QoS resource reservations of IP multicast data flows. We describe the main challenges and discuss the resulting design decisions in detail. Enhancements of an existing NSIS implementation show that the required changes are minimal and do neither affect the unicast protocol operation, nor increase the protocol’s complexity significantly. Instead, all of the advanced features introduced by NSIS, like reliable signaling message transport or support for sender- and receiver-initiated reservations can also be used with IP multicast flows. Evaluation results confirm that the overhead introduced by supporting IP multicast in NSIS compared to unicast reservations is negligible and that the presented solutions also offers scalable sender-initiated reservations.