The global coupling predicts the appearance of one or more clusters where each cluster consists of a subset of synchronized oscillators. The aim of our work is to show the first experimental evidence to our knowledge, of clustering effects associated with synchronization. This was achieved by analyzing the intensity oscillations of the longitudinal modes of quantum dot semiconductor lasers. We find that the modal intensities can oscillate chaotically with different average frequencies and observe that groups of modes may have the same frequency indicating the appearance of clustering. We also observe a highly organized antiphase dynamics leading to a constant total output power and the propagation of perturbations across the optical spectrum.