Control which accounts can SFTP

I need to control which accounts can use SFTP. I thought this was just SSH access but I don’t even seem to be able to do this. The only user I can SSH into my server with seems to be root. I am on Debian 5 and Virtualmin 3.75gpl.

How do I control this and why can admin users for my virtual hosts that say “Email, FTP and SSH” not login via SSH(Putty) with their password?

Shells usually determine this stuff. What shell are these problem users configured with?