In The Infernal Devices
the Pandemonium Club is an organization of mundanes meddling with magic and the Shadow World. It is said to be a rather old organization of mundanes who have interested themselves in the magical arts where, at their meetings, they do spells and try to summon up demons and spirits. They are powerful in the Downworld, their members being rich and important figures in mundane society
In The Mortal Instruments
Pandemonium is an all-ages club in modern New York
It was also a place where Downworlders and mundanes mingled.
Shadowhunters also came to catch demons who sneak their way into the club.