How can people ftp TO my machine?

[M A V I C]

I'm still confused. So you have FTP enabled because otherwise they couldn't see your machine via FTP. And you've somehow configured anonymous FTP. How did you do that? Are you sure you haven't simply granted read access to a guest user?

One way or another, if you want them to have write access to your Mac, you're going to have to setup a user on your machine that they can write with.