how to tell wife i want to share her

any advice?

Just tell her? Is there any other way?

When I was dating my wife we used to watch porn in bed and get extreemely hot and nasty. I had shared my previous girlfriends, but I was a little worried about scaring this new one off so I tried to play it cool. During one of these sessions, during a gang bang scene, she asked if I would like to see her do another man. Inside I was thrilled, but outside I remained cool and said hesitantly "I think it would be very erotic. If you think you could handle it, I think I could." In the heat of passion, many seeds can be planted. 3 months later we were quite tipsy and she invited a friend to join us in bed. After that she was hooked and we have had many enjoyable escapades.

As the above scenario is not exactly what you were asking about, a good porno movie with discussion and stimulation, can loosen up the inhibitions and plant some seeds for the future. Don't rush it though.
If its a drippin', I'm a lickin'