You sound so bubbly and excited - you're making ME crush on you a little bit! ("Sweetie, your NRE is showing."
Take any of this with a grain of salt - may not apply in the slightest! SO much depends on the individuals involved and your relationship with each of them.
First advice - living together can put a strain on even the strongest relationships, adding new relationship dynamics into the mix can cause implosions. If it were me I would simmer in my lovely NRE for a little while until a.) the roommate situation has settled in completely (you may realize that his habit of leaving wet towels lying around is a complete crush-killer for you) and/or b.) your husband asks you why you are waltzing around with that secret little smile on your face...
At that point I would go to my hubs and gleefully confide that you are really enjoying living with your friends but that you have developed a crush on MrFriend and would he be interested in bedroom games with MrandMrsFriend? Or does he think that would be a bad idea given that, since you are crushing on him, you feel that you might develop feelings that are something "more" than swinging (such as poly)? See how he reacts - at that point you could converse further (perhaps he has the hots for MrsFriend, etc. etc.) or let it sink in for a few days (weeks...months) and see if he brings it up again.
Once hubs is on board -
THEN I would approach MrsFriend along the lines of "My hubs thinks you are super hot and I have a little crush on your husband - so we were wondering if you two had had any thoughts/conversations in that direction...." If they have then you can talk about how that could work (including what happens if something turns into more than swinging). If not, then let it sink in for days/weeks/months..see if they bring it up.
Now, you (and others) may question why I would talk to your SO and his SO before broaching it with him. I get the feeling that you are getting "I'm interested" vibes from his direction, so you kind of suspect that he might be game. If you and him have a heart-to-heart and then present the two of them with a "we talked about it and this is what we want" (which is actually an option that you didn't list but happens a lot) you run the risk of blind-siding them with something they have never considered and they may feel "conspired against" or even wonder if there was something already going on.
(My first threesome - recounted in my blog - resulted from MrS's ex-girlfriend approaching me about a threesome - it worked out really well!)
PS. What I would NOT do is what I did when I got involved with Dude - i.e. keep myself in a complete state of denial and confusion, while pushing boundaries and having the whole deal erupt into chaos - which somehow managed to work itself out (like a phoenix from the flames) due to the fact that my boys are the most wonderful men on the planet and are willing to put up with me, the luckiest-girl-in-the-world (who also happens to be, at times, a COMPLETE JACKASS).
(My blog hasn't gotten that far yet -perhaps I am reluctant to reveal my jackass-ery to all the lovely people here on poly.com).