Draco Malfoy's gang was a group of Slytherins who were often the children of Death Eaters, and/or Voldemort sympathisers or Death Eaters themselves. Most of them hung around Draco Malfoy, the son of a well-known Death Eater and wealthy pure-blood, Lucius Malfoy. Most of the members were known pure-bloods, using their status in order to assert their dominance as wizards. At least Vincent and Gregory's fathers were known Death Eaters as well. That may be the reason they knew each other so well, seeing as Draco made friends with them shortly after meeting in their first year at Hogwarts.