I can tell you how I got together with my FWB. So we were hanging out and I told him about my other partner (at the time I had two, since then I've broken up with one of them), he already knew about the first one. Later I clarified that I'm also open to other people besides these two. Then after some time, maybe a month or two, he told me he and his girlfriend had also opened up their relationship. He had some questions about that and we talked quite a bit about open relationships in general. Then one night we were at a bar together and started flirting and dancing and ended up at his place. Then it just developed from there. First it was just random hook-ups, as in bumping into each other in a bar and going to his place, but later we started making plans to see each other and ended up meeting about once in a week or two.
So my advice to you is to talk about being open first and see how your friend(s) react to that. Give them time to digest that info. Then later you could for example flirt a bit and see if they go along with it. Even though I usually like the direct approach in most things, I'm not sure how I'd react if someone said to me something like GalaGirl suggested. I guess in these things I like to see if the attraction/connection is there and if it is, then just go with it. That kind of direct suggestion feels a bit too...rational for me. Like a business proposal.
Me: bi female in my 30's
Live-in partners: Hank and rory
Also dating: Yvonne and Zooey