It's a long process, you need a lot of love and patience with uncertain outcome. We needed three years to finally come to cuckolding. At start encourage her to dress more sexy, compliment her a lot, in compliments ad that gays are looking at her, how hot is she. Make her for start relaxed with the idea that you are enjoying fact that other males desire her. Than, the next step is to tell her openly about your fantasy, chose the moment carefully, and think how are you going to tell her. I did that when we were cuddling, just have to feel when is the right moment. At first, probably, she will say you that she do not like the idea. Don't insist, tell her it is because you love her, because her pleasure pleasures you, etc. Continue with compliments, tell her always when some male is looking at her. If you attend party, leave when some third male talks to you two, after that tell her that you enjoyed watching them talking so nicely. Than, start to send her cuckold captions that are explaining the whole thing. Not those explicit, their are several weary intelligent captions that could be fine. They are a lot serious articles on the internet about cuckolding, show her a few.
Next step is to play games with her, one of the most important aspect of this kind of relationship is that she has to be 1.000.000.% sure that you really mean it, that you not going to go mad when it actually happens. I started with questions about exes, did she enjoyed sex with them, what they did etc. and clearly show her how it turns you on. Ask her which actor is sexy to her, offer her to go down on her wile she is watching a movie with this actor and encourage her to fantasies sex with him. Invent imaginary cuckold situations while she is giving you a hand so she can see how are you going to explode. Ask her if someone flirted with her during the day, show how happy and trilled you are for that. Encourage her to go out with her friends, to dress sexy for those occasions, by her sexy lingerie and tell her to ware it when she is out, etc.
So, it is a journey of a many, many small steps made carefully with love. There is much more, what i wrote are basics, you will find on internet. I don't think that taking her to swing club and openly offering her friends or immediately taking her to swing club will do the trick, quite the opposite. To come to offering her friends or going to swing club is a long road.
P.S. When you tell her, make sure to be crystal clear that you are not asking that because you want the same for you, that she will be only person in your relationship free to have sex with who ever she wants.