Formscentral doesn't have any password protection for forms at this time. Our best suggestion for controlling who fills out a form is to require a user ID and embed the form on a web page that controls access.
Thanks Andrew, are you talking about .htaccess? I'm using Muse to build the page and hosting the form via an iFrame.
There are several ways to do this. As I am not familiar with Muse you might try posting your question to their forum.