Well, you either can use the many boards we offer, or you can write via our PM system, but you will have to make sure your potential writing partner wants to write via PM. ~nodnod~
As for the threads - each thread is private, in fact, between yourself and your writing partner. Nobody else will post in a thread they are not a part of. Everyone is free to read, of course, but not comment in the thread or even make a post. That's a no-go here on E.