Let's get realistic; nothing is safe on the internet, period. Just being a premium member and posting to only groups/connections does not protect your work. For less than ten bucks, anyone can join for a month and have access to all posted work of premium or basic members and it's doubtful even the private groups would turn down a potential new member should they ask to join. Ten bucks is not going to really thwart anyone so inclined; it's probably cheaper and faster than hacking the old fashioned way.
As Linda pointed out, the automatic copyright on site protects your work as well as anything and can be used in court to defend it and go after anyone who profits from it. Of course using a pen name is a no brainer should your work become famous and someone starts to Google your name to find non-public info. If I Google my real name, none of my writing shows up since I don't use it, at least in the first million or so entries and should someone want to put that much effort into finding it, then there is probably nothing which can prevent it, and certainly no constraints on this or any other writing site.
Common sense is our best defense, but yeah, I wouldn't mind paying the lawyers after the fact that someone else has made millions from my work when so far I haven't been able to get a traditional publisher or movie mogul interested as the original author. If someone else can knock down the doors, then all they need do is come to me and I'll gladly offer an agent's commission for their efforts.
So, yeah, we can worry ourselves sick over the possibility of our work being hijacked or we can accept that absolutely nothing is safe from prying eyes on the internet and deal with any future stolen property rights with the courts on our side should it come to that. Just my opinion which has been stated in similar forums before this one. Take care. Vern