While the build-up of large structures, such as galaxy clusters, from the earliest epochs to the present day has been extensively studied with cosmological simulations, the early formation and evolution of their galaxy content, and its relation to the transformation of the host environment, are still somewhat poorly understood. The redshift range around z=2-3 is then particularly interesting as a formative epoch where the first quenched galaxies appear and the most massive galaxy overdensities acquire the distinctive characteristics of galaxy clusters. Processes considered as crucial for the formation of the passive population in field and dense environments are thus thought to occur, and be most easily studiable, at this stage. Recently, a series of discoveries in this previously inaccessible redshift range have started to bridge the gap between the massive virialized structures of the last 9 Gyr and the vigorously active protoclusters of the early Universe, finding intermediate« structures that allow us to study the coevolution of cluster galaxies and their host structure very close to their formation epoch. This still ongoing breakthrough has been made possible thanks to sophisticated space-based instruments and in particular near-infrared spectroscopy with Hubble. I will discuss the formation of the galaxy population in the most distant »true" cluster confirmed so far, at z=2, as well as in the field, and examine the possibilities offered by future missions such as Euclid and JWST.