Now that you mention it, I agree that she was addressing the party. The idea of Griever would have probably been unique to Squall, however. Griever probably wasn't "the most powerful of things" in, say, Quistis' mind, or Selphie's. You're right in saying that Ultimecia would have Drawn from the whole party; it's just that I would think the Griever template would have originated with Squall.

The dialogue Squall cited is actually the 48th post of this topic. I would have quoted it, but it takes up a lot of space.