Group consensus problems in networks of dynamic agents are addressed. We consider the case that communication topologies in a network are switching and the switching occurs among finite topologies. By using double-tree-form transformations, the group consensus is proved to be equivalent to the asymptotical stability of a class of switched linear systems under arbitrary switching signal. Moreover, we study the group consensus problem for the networks of dynamic agents with both switching topologies and transmission delays, some analysis results are derived by the similar technique. Simulation results are worked out to demonstrate the effectiveness of the theoretical results.