The "sign-in" before downloading is not built into Zenfolio (or at least that I was able to find), so I have two different pages set up. They both have identical layouts, one links to the other (with the sign-in prompt) but that page requires a sign in. Both have to have public access, but you control access through the menu.
Preventing Google from indexing the page requiring a sign-in is a good idea to keep people from bypassing the menu. This can turn into a problem if most people use Google to find you website and they end up "inside" your website through a Google search or following breadcrumb trails back somewhere you don't want them to go.
Jay