People know about it vaguely, but it is unfortunately invite only. The only people who can come are those marked as lords of the court OR those coming as their guests, which Luke could do if he could find someone to take him. Basically, members are marked by a white serpent tattoo that allows them access to the location the Midnight Court is held. You can't get through the wall to let you in if you don't have the tattoo or are not with someone who has it.