Well I would buy (unless you already have)
Sex toys. Realistic cocks and role play. Have her ride the cock on the floor while she sucks your cock. Ask her how she likes it and ask if it turns her on. You can do other things like have her wear a blindfold and pretend she’s being done by you and another guy. Have her suck your cock and the realistic one, then lay her down and fuck her with the cock while she sucks on your. If you keep doing these role play she’ll be so turned on by the thought in no time
We are finding hard to find a couple we can have a social relationship with, where my wife and the husband are able to enjoy an ongoing exclusive sexual relationship with each other and their own partner only. It would be much safe for a husband interested in an other woman or this life style and a wife that had no interested in being with an other man sexually.
We started off with pillow talk during sex and moved onto toys. Eventually we took the plunge and she really enjoyed it and I enjoy sharing her. We're still experimenting where MMF is concerned and we still want to try other things as well. MMF can be what you make it.
Always remember that communication is very much the key when sharing your wife. We've always ask each other if we had fun, what we liked, what we didn't like, so we can take that with us on our next adventure. But if you start asking your wife to compare you with the dude you shared a bed with, then that's when jealousy and doubt can set in and it's a recipe for disaster not only moving forward in the LS, but in your relationship as well.
You also have to make sure that the dude understands their role as well. And come up with a set of rules for both of you and the person you share your bed with.