In this thesis the theoretical and experimental treatment of the dynamics of a large ensemble of two-level systems that are equally coupled to a bosonic mode is presented. The connection to superradiance in open quantum systems is explored. The experimental system is based on a 3D cavity with a reduced mode volume and an ensemble of Nitrogen-Vacancy centers in diamond. With this system a coherent population inversion in a macroscopic ensemble is reached for the first time. The experiment is shown to agree well with the semiclassical theory.