Yes, unfortunately, that might be one drawback of posting a story on here. I can not promise you that no one will copy it, or steal original ideas. Of course, I would never do that, but this is a public group where anyone can read your story if it is posted. So please, only post your story if you are comfortable with it. I really hope there isn't a big issue about this, but I definately don't want anyone to steal your idea, so only if you want to.
I'm eagerly waiting for your first post with notepad in hand.. lol just kidding.
If someone does try to steal your idea posting here will actually help protect you because it's timestamped and public. Short of copyrighting your material publishing it publicly with the date and time recorded is the best way to protect yourself. Copyright is about being able to prove you created the original work first. But I totally understand if you'd still feel uncomfortable about it, I feel paranoid so much that I don't let anyone know the working title of what I'm working on because I'm worried they'll deduce the entire plot from that and write it before I finish. lol. It sounds silly when I say it out loud but It's a real concern for me :/