The gradual infall of small dark matter halos onto larger ones has become a relatively straightforward aspect of the standard hierarchical formation paradigm. What happens to the baryons they contain, however, is less well understood. Of special relevance are the processes that regulate and ultimately suppress star formation in galaxies in the early universe and give rise to the quiescent galaxy population. The z=1.5-2.5 epoch is t hen particularly interesting as a transition period when global star formation in the universe starts peaking but also where the first ostensibly collapsed and virialized galaxy clusters appear, along with segregated galaxy populations. From a theoretical point of view, the mode of gas accretion in massive halos is also expected to change around this time, switching from a cold to a hot phase and affecting the build-up and evolution of the galaxies they host.

A lot of effort has thus been devoted to the search for high-redshift structures, in particular galaxy clusters, through a variety of methods. However, as the limited area for which deep datasets are available remains relatively limited, only few massive z>1.5 structures have been found so far. Here I will focus on the regulation of star-formation in medium- and low-mass, X-ray detected halos at z~2 and its implication for galaxy quenching at high redshift. In particular, group-size halos, which are vastly more abundant than massive clusters, allow for true statistical studies and offer a novel way to probe environmental effects in this transitional epoch.