Dynamics of a complicated quantum system interacting with open decay channels is treated by means of the discretized effective non-Hermitian Hamiltonian. The main subject is segregation of collective short-lived resonances, which are similar to the coherent Dicke states in optics. The analysis is carried out with use of two complementary representations (internal and doorway). The phase transition between regimes of weak and strong continuum coupling is considered. 32 refs.