I agree that it all starts 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 she is very jealous and didn't want me playing with other women. My wife didn't think it was fair for her to play and me stay faithful. It just took time. I was consistent in my communication and support for her giving it a try. Eventually, she met a guy who was too hot to pass up and she took the leap. In my experience, if you really want this to happen, tell her how you feel and then give her time to get comfortable with the idea. At some point, she will have the opportunity and may just take it.
Fast forward a few years and she has played with 22 different men and 35 total sessions with them. There have been quite a few lessons learned so be careful what you wish for. Please accept that regardless how much you talk about things, rules will evolve over time. 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 fucoses on the enjoyment for her and her friend then pays the consequences later. We also learned that she would rather play without me there. At this point, 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. My advice is to accept 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!