Bad Place architects design their neighborhoods at desks in a large open-plan office, and there are also rooms which can be booked for meetings. You can find a wall titled "Employee of the Bearimy" which seems to contain all the best torturers of the bad place, such as Micheal. HQ also contains The Museum of Human Misery which includes an exhibit depicting Neighborhood 12358W and featuring animatronic representations of Eleanor, Chidi, Tahani and Jason. They notably give "sexual harassment training" in which they teach the demons how to sexually harass humans. There is at least one portal (and likely more) leading to the Neutral Zone.
Shawn has a senior position at HQ, and Michael worked there before designing and moving into Neighborhood 12358W. Other characters who have been seen at Headquarters include Val, Toddrick Hemple and Glenn.