I can understand how you feel. In a way your trust has been broken, because what you thought you knew about each other, you didn't, you were out of the loop. You don't all share something, they have a little something more. You're clearly in tune with social dynamics- like you say, it changed the group dynamic. And you're right to feel wary about moving in with them. Sleeping with housemates is 90% of the time a big no, and being in that environment will definitely be noisy (LOL) but also, potentially full of tension. But it doesn't have be the end, your friendship could get better. You need to bring it up with them- like you would have thought they would with you, just because it would have come up.
Do you know why they kept it a secret?