I just wanted to say, welcome to our forum.
Does this couple know anything about polyamory? Do they know that you and your boyfriend have been in an open relationship, and what that means? Do you do a lot of talking with them? I know you've said you've been hanging around with them a lot.
Perhaps one way to start would be to tell them you're polyamorous, ask them if they're okay with that, and if it's something they'd ever consider. I can see how it's a difficult conversation to start, but it's one you'll want to start eventually.
Can I ask what's bringing your relationship with your boyfriend to a close? I'm just wondering.
I imagine some heavy-duty talks are in order all around. I hope Polyamory.com is a helpful resource to you in these matters.
Love means never having to say, "Put down that meat cleaver!"