A few of Seamus's friends now know. Well, three of them.
The first one game him a high five, the second one said "good for you", and both were pretty much "meh" about the whole thing, in that they don't seem to be very curious or interested, they just take it for what it is. They both have known for a while that we're polyamorous even though neither of us had other partners, so I think they expected it to happen at some point.
The third one, the one who took Seamus's keys away when he was drunk and had him sleep on his couch, was a little bit different. Seamus told him he was going to catch a movie with the first two friends + Kay, and the friend said "Oh, right, we still need to talk about that".
What he meant is that he told Seamus on the day he was drunk "when you're sober, we'll talk about what you want me to do in cases like this, but right now I'm not letting you make that decision", meaning the decision to go home with Kay.
So Seamus told him "you don't need to worry about it anymore, because Kay and I already made the decision". His friend immediately worried about me, asking if it was over with me. Seamus said "Noooo." and then added that I was "fine with it". He regretted his choice of words afterwards (saying "fine with it", we both feel, makes it sound like it's not something we decided together, but something imposed by the other that we suffer through. It doesn't convey that I prefer him being with Kay rather than not). But he was in a hurry and it was a simple way to say it at the time.
Seamus says it was a little bit awkward to go to the movies with Kay and the two friends who know about them. He didn't say more so I don't know if they were obvious about it (holding hands, kissing) or not at all. I think he would have told me if they did so I assume they were mostly friendly.
He didn't go back to her place, first because he was exhausted, but also because he wants to make sure she doesn't think he's using him, so he doesn't want to have sex every time they go out together. He wants to make it clear that they're still friends as well, among other things.
Oh, and every one who knows was told after Kay's approval, and they're Seamus's close friends. It seems she's more worried about their mutual friends/acquaintances who are closer to her, not to him. Still, I wouldn't be surprised if the PDAs were limited due to her still wanting the relationship to be low key.