The Writers' Lounge

Eh, you only discover that you're mostly safe by interacting with writing comms. Which, of course, you can do without sharing work. (Although it's often hard to get a beta without doing so, as betas often don't want to agree to read more than a very little bit unless they're fairly convinced that they won't be agreeing to a novel worth of misery.)

You can definitely sue if someone shares your copy righted work without permission. From the Critters Website:

Absolutely not! No way! Big trouble awaits!

All works submitted to Critters are copyrighted, and you have no authority to share the work with anyone, at all, period. And if you have the urge, keep in mind that the DMCA (Digital Millennium Copyright Act) carries really stiff fines (minimum $2500 -per violation- up to millions of dollars) and up to ten years in jail. Yeah. Heavy stuff.

Don't mess with the copyright law. (Note that maximums are unlimited as far as fines go.)

This, of course, relates only to the US. But similar laws exist in most other countries. So if you shared something with someone and they spread it all over (or pretended it was theirs), you could technically sue them under this law for any damages. Of course, you'd have to figure out where they were (which could be tough) and prove that it was yours (pretty easy, as I'd assume that there'd be an enormous digital trail).

And this really wouldn't apply to fanfiction, seeing as your legal rights to it are nebulous there at best.

But if someone stole your original fiction, you could definitely sue and get them into a huge amount of legal trouble. (And this is beyond finding where they shared it and getting them into trouble there, too. I suspect that magazines, etc. don't look too fondly on plagiarism.)

Regardless, the penalties (both legal and also, do you really want to be known as the person who stole someone else's work and pretended it was yours?) are significant enough that most people tend to be pretty good. No guarantees - of course - there are none in life. But most people's fears are rather over blown. It's not that hard to prove that your work is your own. It's not that hard to make a plagiarist's life miserable. And most people really aren't out there to steal other people's stuff, anyway, as they're having a hard enough time promoting what they write themselves.