Just a few quick thoughts:
I'm not one for strict definitions, but usually "poly" is short for "polyamorous", meaning there is
love involved. What the OP and others have described, a group of friends who trust each other and are very comfortable, doesn't sound like it gets to the poly level of feeling. Just my little nitpick for the evening. No offense meant.
I agree that something like this group arrangement would be difficult to achieve, at least on purpose. The best I think any one couple can do is introduce their playmates to each other, having discussed the possibility of having this kind of arrangement, and hope they like each other. We can also ask to meet the friends of our friends.
If a group does get going, I think it's best not to require folks to stay "committed" to the group. I anticipate that would be just asking for trouble. Another poster mentioned that in their group, at least one couple committed to the group and then "cheated" on them. I think people should just be honest with each other. We can't stop people from doing what they're going to do; we should just make our decisions about who we ultimately want to be with and who we don't, based on their behavior and mutual attraction.
If there are more than three couples involved, it's natural that some people will want to play in certain combinations more than others. (Actually, if there are more than
two people involved, that probably holds
) Some people will just have chemistry with some group members and not others. As long as there's not a huge inequality involved, and no one is getting shunted aside, I think it's best not to keep score.
Expect that little jealousies will emerge. When we introduce our friends to others, I am always concerned that they'll like each other more than they do us, and then we'll be standing there watching while we lose them both. It's natural for people to be very excited about "the new kid in town". I've been jealous before. When I've overcome these feelings, the key has been feeling secure that my playmates will still want to be with me.
I can't say we've developed a group yet, but maybe in the future. I think if we were too focused on that, we'd be setting ourselves up for disappointment.