Amplitudes for exotic states at JPAC

We discuss the activity of the Joint Physics Analysis Center in building models which satisfy S-matrix principles. In particular, we show some results on the analysis of the $\eta \pi$ exotic resonances at COMPASS, and we discuss how to extend our formalism to photon beams to be used in the GlueX experiment. We also discuss a model to fit the $J/\psi p$ spectrum measured at GlueX, of interest for the search of hidden charm pentaquarks.