There is a lot of good advice already on this thread.
We love it when a guy says hello and talks to us but sadly it is all too rare. Most that we see just hover around waiting for a couple to to make a move to the play rooms, then follow in the hope they can join in with something. Sometimes it will work, but more often not.
Say hello, compliment the lady on her outfit, and smile. If your advance is welcome then you will soon know. Of course it won't always work, some will have no interest but in our experience the couples that are looking for guys will nearly always welcome it, even if it doesn't go further.
Another question we get asked regularly is: "How do I know if a couple is looking for a single guy?"
The answer is very simple. Ask them, they can do nothing worse than say no, but you will be surprised how many will say yes.
Last but not least. If you do get talking to a couple and would like to go further then ask the lady. I've lost count of the number of times someone has asked me "can I have sex with your wife?"
My answer is always the same. Don't ask me, ask her.