I have seen and experienced it happening. For us, it was a year investigating and exploring the local swinging community. Our best friends (a couple) found us out and then became interested in the LS. We all went out one night to a local event and ended up doing the same room / same bed thing. After that, we all started playing together. It turned into poly real fast as all of us already had strong feelings as best friends. Unfortunately, both of us (both couples) never had group sex or group love before and we all fell into our own little insecure emotional traps. Each one of us got scared of how powerful the attraction and emotions were and we all lost sight of the big picture and most of all, communication. They are still our best friends and I don't know if we will ever go down the poly road...or swinging road again. I can tell you however, it was the most incredible time of my (our) life.
I've seen other poly folks that have gone as far as to join houses and be one big family. Some are great, some become "War of the Roses" doubled!
Poly, like swinging can be a wonderful thing, just remember it's all dangerous. If you are a hard core sport-F'er swinger, it can be physically dangerous, and if you're a committed pure poly person, it can be emotionally dangerous. One thing's for sure, it's not dull!
G