Don't want anyone to see your background? Just send it to me, or tell me the general gist of it. No one gets to see unless you want them to. IC-wise, was a character there for the events in question? Otherwise, they only know what they heard. Work that out between players if you want them to know anything extra.
This extends into the game proper. Use of private tags will be frequent: You only get to see what your character is there for, and hear the rest, if that. This is, in part, because the party is under no obligation to, as the game unfolds, stay together or even be on the same side. Obscuring half of what goes on in the threads will help to make this work better.
That's all for now.