Well, here goes. We're going to try to explain it as far as we know as we've not got much experience in these matters. Any other more knowledgeable doggers please do contact us to change or add to any of the following.
Usually you have a situation where couples are in or around their cars, having fun for all the voyeurs who've turned up to see. Sometimes these voyeurs are asked to join in.
Only join in or move closer if you are asked, or if you see the interior lights of the car flashing (a signal to move closer as the couple want voyeurs to watch or perhaps even join in).
"There are laws that protect members of the public from being unwilling witnesses to sexual behaviour or from being harassed or intimidated in their use of public spaces or facilities. So sex in public toilets, parks or places where other passers-by may be offended is illegal."