Couple number one: Secured.
We've never had sex with them, for good reason - it's the wife's best friend and her husband. Not exactly "off limits", but still kinda weird. So assuming there are more attendees than just the four of us, it's good (the wife wants to be able to keep her distance from the best friend, at least while the action is happening - harder to do if there aren't more people there).
Couple number two is interested, but we're waiting to see if they can make it down (we have a lot of people from Northern California on our "to do" list, but the party's down here). Our local friends are still a "maybe" - I don't want to bug them, but it'd be nice to get planning.
I'll be talking to couple number four soon, and while I'm sure they're interested, I'm not sure they're available. Still considering whether it's gonna be all couples or some singles too - guess it depends on who can make it. But so far, so good! If everyone's a "yes", that's 12 people, pretty awesome. That's not even counting any friends-of-friends who may get invited.
If you're reading this, and I know you, and you're interested, let me know!