  We study a model describing competition of interactions between $N$ two-level
systems (TLSs) against decoherence. We apply it to analyze dye molecules in an
optical microcavity, where molecular vibrations provide a local source for
decoherence. Most interesting is the case when decoherence strongly affects
each individual TLS, e.g. via broadening of emission lines as well as
vibrational satellites, however its influence is strongly suppressed for large
$N$ due to the interactions between TLSs. In this interaction dominated regime
we find unique signatures in the emission spectrum, including strong
$\mathcal{O}(\sqrt{N})$ level shifts, as well as $1/N$ suppression of both the
decoherence width and of the vibrational satellites.
