I would bring it up one night while having them over to play cards and have some drinks.
I would wait till everyone was feeling good and figure out a way to add it to the conversation. Vanila couples talk about sex all the time in adult get togethers, although the talk is a little tamer than a swinger get together
. Just bring it up in a humorous kind of way and gage their reaction.
You don't have to bring it up as swapping with them specificaly but swinging in general.
Who knows.....