I'm trying to understand the best way to allow for public pre-order while providing access to a select few testers. Do I need separate projects for that? If I make the project public and post executables (rather than a pre-order file) then won't the executables be available to all who access the project page?

You can create download keys for testers so they don't have to pay for the game. If you want to open up preorders that don't have access to those testing files, then yes you'll probably need to set up 2 projects. One feature I'd love to see is more in-depth control of keys, for instance, "Allow Access to Hidden Files." I'll make a separate post about this.

There's no way to do this right now, since a project page only has one state at a time. I recommend doing pre-orders on your final page, and then distributing a tester build on a separate page that is set to restricted. Tell me if this doesn't work for you.

Thanks for the replies guys - I'll go with @leafo 's recommendation.