Riso, posting a story on an open forum leaves it open to be stolen. It's a chance you take. Look for forums that are closed. It lessens the chance of stealing. Anything you write is automatically copyrighted to you. It's if you need to prove it that's work (sometimes). You can get your work copyrighted by the US Copyright office. It costs money.
Also, if you're looking to publish your work, you may want to be wary about posting in an open forum. Many editors are not accepting work that has already been in the public eye.
I'm glad you found a writer's group. I wish I could find one in my area.
I love the workshopping site I go to. It's more the community of people there, really. Good site, good works, honest crits. It's just unfortunate it's an open forum. So I keep my serious work in the closed groups there.
I'm not sure about the set up of the forum you go to. If you can put a PDF in the site. Then again, I'm not sure how it's done there. Is there a body box, title box and so on...
Another place you may want to check out is writers.com. I believe it's run by Sci-fi Digest. They work with short stories and novels. It's a closed forum and you are promised so many crits. They also work with Fantasy and Horror.
Good luck on your work.
Lila