Hi, and welcome to both of you. I'd just like to add my reassurance; while I suppose it's possible that a piece of writing might get copied by someone else, in practice I have never heard of any such thing happening. The benefits of a critique group (web-based or face-to-face), far out-weigh the astronomically minimal risk of having your work copied.
A couple of things you can do on WW, to limit the risks even further, is
1) when you upload, set your work to 'viewable only by group members)
2) don't upload the whole thing. Half a play is no good to anyone.
If you're still worried, imagine what would happen if you, yourself, tried to steal someone's work off the internet. Leaving aside any moral considerations, it's just not that simple. How do you find a piece of writing you're sure will be accepted by a publisher? (Not so many of those kicking round on the web...) How do you work that piece of writing into your own novel? Or if you try and lift the whole thing, and by some unimaginable stroke of luck get it published, what happens when you get asked to write the sequel?
'Hooks', or 'ideas' are theoretically easier to steal, but really - honestly - there are no original hooks. Think of one and I'm sure you'll find five books that have done it already. Vampires? Tick. Pirates? Tick. Vampire pirates? A Tick. An idea is nothing on its own, it's the execution that makes it any good.