We've had drinks in cabins on site before, head office has never geographically been even worthy of even considering for such an event. Wouldn't have been difficult to arrange for 15-20 people to work late one day and do whatever you want there though, pretty much everyone would be gone by 6, cleaners done half an hour after that.
Not sure there'd be much interest in having in a party in a poorly maintained office building in the middle of nowhere that just have looked 10 years out of date before it was even finished though.