Many cloud-hosted applications and cloud infrastructure management services with publish/subscribe semantics rely heavily on the use of group communications. Although multicast is useful for efficient group communications, traditional IP multicast has seen very low adoption in cloud data center networks (DCNs) due to issues with its stability and scalability. The introduction of Software-defined Networking (SDN) in DCNs has provided new opportunities for re-engineering and effectively utilizing multicast capabilities that can overcome existing impediments to the adoption of IP multicast. To that end this paper presents an adaptive and flexible middleware solution called SDN-based Multicast (SDMC), which provides group communication capabilities in DCNs in a way that is both network load-aware and switch memory-efficient. Applications using SDMC remain agnostic to the underlying group communication semantics, which SDMC provides efficiently by dynamically adapting between unicast and multicast in accordance with changing network bandwidth and switch memory usage.