ANSWER:

You can’t.

sftp logs you in via ssh – there has to be an account that you log into.
You can share the account (create multiple keypairs, or just pass out the private key to everyone who needs access), but there is no equivalent of plain-old-FTP’s “anonymous” user and no password.

You could use FTPS (Plain old FTP, wrapped in SSL) to do this though.

by voretaq7 from http://serverfault.com/questions/381681