This paper deals with open queueing networks with group arrivals in continuous time and discrete time. By using the method of entropy maximization, an equivalent single customer arrival process is obtained and the results are then applied to study a variety of queueing systems. As extensions to queueing networks, the following cases are investigated: single-server nodes in continuous time and discrete time, multiple exponential-server nodes in continuous time, and single exponential-server nodes with finite capacity in continuous time. Several examples are included for illustration.