I'm certainly not positive that it's a goldfinch, but it still seems like an American goldfinch to me. Unless it was significantly larger than the goldfinches, size can be very relative. Also, black beaks can be found on American goldfinches in their non-breeding plumage. You can go to Cornell's site to see pictures of some with dark beaks. I have to admit, though, that part of me thinks its a goldfinch just because it was seen with other goldfinches, not exactly a definitive ID. Maybe someone better with orioles can weigh in on this and give you a 100%-sure answer.