Dogging is a British euphemism for engaging in sexual acts in a semi-public place (typically a secluded car in a car park) and then watching others doing so. Frequently, there are more than two participants; both group sex and gang banging can be included. As observation is encouraged, voyeurism and exhibitionism are closely associated with dogging. The two sets of people involved often meet either randomly or (increasingly) arrange to meet-up beforehand over the Internet.
Would love to hear from anyone who has done this.