Group things...

I'm aware that among most people of my age, and probably also most people on this forum, this is something uncommon, and also perceived negatively by many. Getting off with someone met at a party is one story.
Getting off with many people at the same time is somehow strange.

Anyway, I'm not asking for moral opinions on this, but rather:
how/when/what circumstances usually lead to such experiences?

I hear couples in their thirties talking about "swinging", but I know that there are also real (not just erotic) relationships between more people at the same time...

I've also heard about real "parties"... personally I have never attended them, and I'm honestly not sure whether they actually happen in real life (and not just in adult entertainment)?
I hear rumors... but rumors are rumors...

What do you know about this?