In the end, it was decided that Bill would accompany them to Gringotts due to his knowledge of working there as a curse breaker. He had convinced them to leave the goblin from the Malfoy dungeons out of it. He would only betray them. Goblin loyalties were different from wizarding loyalties. Potter would join because he was freaking Harry Potter and left nothing to anyone else. Draco would go for obvious reasons to get them inside the place, to begin with.