I really believe the way to get your wife into fucking with other men is to let nature take its course, and to not try to manufacture a situation which may nor may not happen.
Maybe you can invite your friend, or friends, to your house and have your wife give them a sexy dance. If she's brave enough to strip for them, and they get a look at her nakedness, then they will tell you, in one way or another, if they would like to fuck her.
If she, and they, are willing to fuck, let them decide how and where it will be. If they let you stay and watch, fine, but don't push it! The important thing is to get them both into the mindset of fucking together. After that happens the first time, they may be ready to open up a bit and let you watch, or even participate.
Sharing your wife's pussy with other men is a sensitive matter, and there are no reliable instruction books on the subject. If your wife is attractive, and is open to the idea, there are lots of men out there who would love to fuck her. The trick is in setting-up the right situation with them and her.
Have you ever looked a friend in the eye and asked, in total sincerity "Would you like to fuck my wife?"