I agree that it all starts with communication. That being said, not all wives will be open to this, no matter what you do. My wife would have opened up sooner but was very jealous and didn't want me playing with other women. My wife didn't think it was fair for her to play and for me to stay faithful. It just took time. I was consistent in my communication and supported her in trying it. Eventually, she met a guy who was too hot to pass up, and she leaped. In my experience, if you want this to happen, tell her how you feel and then give her time to get comfortable with the idea. Eventually, she will have the opportunity and may take it.
Fast forward a few years, and she has played with 22 men and had 35 sessions with them. Many lessons have been learned, so be careful what you wish for. Please accept that rules will evolve regardless of how much you talk about things. As your wife or gf gets comfortable with this activity, she may become more "flexible" with the rules. At one point or another, my wife has broken every one of our rules. She has always been honest with me when the rules changed. (Though usually after the fact) We found that she gets caught up in the moment and focuses on the enjoyment for her and her friend, then pays the consequences later. We also learned that she would rather play without me there. Even though I would prefer to be there, it is a take-it-or-leave-it situation. She hasn't included me in her playtime in nearly two years, and out of her 35 sessions, I have participated only twice. One other time, we had a suite, and I was in the other room while a new friend fucked her. I advise accepting that it is your wife's body, and she will have the final say in how things happen. (No, I am not a cuck, very Alpha in every way). I am interested in hearing how other hw husbands have found common ground without squashing their wife's sexy play.
Best of luck!