hey, i haven't really been on here in awhile, so i didn't see your post until now. i think when you initially post the story, they give you an option to co-write it and you just say the name of the user you are co-writing with. i hope this helped and that you are doing well!
Hahahha, same! Ah, I forgot my username AND password a long time ago D:
And it suddenly came to me(:
I changed my username, so that may be why you don't remember(: