NYNET (ATM wide-area network testbed in New York state) Communication System (NCS) is a multithreaded message-passing tool developed at Syracuse University that provides low-latency and high-throughput communication services over Asynchronous Transfer Mode (ATM)-based high-performance distributed computing (HPDC) environments. NCS provides flexible and scalable group communication services based on dynamic grouping and tree-based multicasting. The NCS architecture, which separates the data and control functions, allows group operations to be implemented efficiently by utilizing the control connections when transferring status information (e.g. topology information, routing information). Furthermore, NCS provides several different algorithms for group communication and allows programmers to select an appropriate algorithm at runtime. The authors overview the general architecture of NCS and present the multicasting services provided by NCS. They analyze and compare the performance of NCS with that of other message-passing tools such as p4, PVM, and MPI in terms of primitive performance and performance.