I think that I have posted before on a similar topic. A relatively simple answer is: role-play. We started with a dildo while having sex, just to give her a glimpse into how it feels like having 2 cocks inside her and how it feels for her to satisfy 2 men at the same time. It was obvious that it was a turn-on for her as well. Every time is really fun for both of us...After that, I started to propose the name of a friend of mine while having these role-play games....The fact that it was a "real" person was making everything it so much more interesting...
At this point we were having strong fantasies into trying something with this particular friend. I proposed her to wear short panties with no underwear in his presence to see his reaction...You can see just an example of this type of outfit in the pictures below (not my wife). Myself, I was having an intense pleasure when she was walking like that in front of him, bending over in front of him. I could feel his desire of wanting to fuck her. He was also a little bit uncomfortable. So, we have tried these types of role-play games between us and these types of exhibitionist games in his presence until one day....In this day, he was at our place, having a few drinks. He was staying the other side of the bed enjoying his beer. My wife was in her "traditional" shorts, also on the bed, lying on her belly.
At some point I started touching and playing with her ass checks...I asked him if he was bothered by this. He said no...And the...the magic happens...