My wife would simply tell me if she wanted to have one of my friends fuck her. Then I would invite him over one evening, and she would take it from there. Usually she would wear one of her shorty shifts with nothing underneath, and once she had him looking, would ask if he would like a little pussy. Very few ,ever said, "NO." I would have the pleasure of watching.
As a husband who has experienced this many times over many years, I heartily recommend it. There's just nothing better than sloppy seconds and fucking my wife immediately after someone else is always, always, wonderful - she is so alive and her sex-drive is off the chart, constantly. Greatest thing ever.