Dear Miss Snark,
I write suspense novels and would like feedback on a couple of books I've finished. I fear, however, that if I join a writers' critique group, my story concepts might be used by someone else looking for a good plot. The very uniqueness of a suspense plot is, in my opinion, what sells a suspense novel.
Why, one of your snarklings even 'fessed up to this by commenting that the Happy Hooker Crapometer gave her some good ideas! (They weren't mine; I was afraid to participate because of above reasoning, though I certainly would have loved your feedback.)
What to do?
First, you need to realize that while plot is important it's the writing. You don't have copyright on ideas, only execution (ie writing).
If I seriously thought there was going to be a problem posting people's work in the crapometer I wouldn't have done it. People get ideas from all sorts of things including previously published work (T. Jefferson Parker acknowledges a debt to Jon Lethem in a recent novel); art; music; and, their own families. It's what you DO with the idea that counts, ESPECIALLY in suspense novels.
I find the people most paranoid about other's stealing their work are the ones least likely to be stolen.
Screenplays are a different story, but you've said novel.