I think I'm going against the grain, but this proposed arrangement might just blow-up in everyone's face. Stop me if I'm way off base here, but you're proposing a relationship with the husband of a couple you and your hubby now live with? I think there's a lot to consider here.
It's going to be your friend and your hub that you have to worry about (that's 2 people who's reactions are going to matter, living with you every day). If either of them react badly to this, then it's not going to go well. Just both knowing what you want, even if no one goes forward with it, could put everyone in an awkward position.
Can I suggest waiting until there's more physical space? Let me explain.
What if you confide in your husband your desire for his friend (and say that works ok, a big 'if') and then you talk to your girl friend and say that does NOT go over well with her? What then? It's not like you can unsay all those things, nor can you go home to get away from her (they live with you!).
Making one person understand that you're poly (when their reaction is unknown) is one thing, but hoping 2 people agree to this? It's twice the risk. And I'm not even thinking about the hubby's friend yet (that's another unknown). I'm a risk taker (I broached the subject of poly to my mono wife just in March), but this scenario you're proposing seems like a huge risk. Hope the result goes the way you want it to.