First Welcome to the board.
Like Alura said you'll get lots of variations of how to bring it up. Alura hit the first and most critical element that your marriage needs to have and that is communication. If you don't have that you need to work on it first. Open, honest, and trusting communication is vital.
Also, BradandJanet mentioned bringing up and sharing fantasies together. If she is not comfortable with you in the intimacy of your bedroom to talk about her fantasies do you really think she'd be comfortable thinking about having group sex with strangers?
Mr Spoo and I had/have a great sex life before swinging ever came up. We also enjoyed friendship where we could be ourselves as far as being flirtatious and open about sex (i.e. not a prude if someone says something about sex
).
We already had all the key elements needed for him to bring it up. He asked me one day walking across the parking lot after work
"Have you ever heard of soft swinging?" This might be a good route for you to go. It's less scary to start talking about having sex with each other but with another couple in the room doing the same thing then it is to think of a pile up of people having group sex.
You can't just pounce on your wife with the news that you want to check out swinging. For those who really don't know what it is about envision something WAY out of their comfort zones (i.e. big orgies).
Take it slow and then once the bug is planted steer her to this board. Let her read, question and research it herself. Be prepared to accept that it just may not be for you. The fantasy become reality is not always as you would expect.
Take it slow and give her time to be comfortable.
Good luck
Mrs Spoomonkey