Sex and the Office Girl (1972)

Sex and the Office Girl purports to show what happens behind closed doors during a late-night office crunch session. Forced to spend the evening at work catching up on filings for a big new account, the office pool quickly finishes its duties but, unlike a typical workplace where everyone would be dashing for the door, decides to stick around and - in true 60s fashion - have a nightcap and an orgy.