IIS two layer authentication for BETA webapp

I have an ASP.NET MVC webapp that has a public area and a member area with login using cookie + oauth, nothing fancy is just a regular webapp.

I want to publish a BETA version for testing but I don't want it to be publicly accessible, something like a Basic Authentication before entering the website would be enough. Cant do authentication by IP or domain because the testers will be anywhere.

Is there any way to activate this without interfering with the website login?

I am aware that I may configure a reverse proxy in front of IIS to handle the authenetication but I was looking for a simpler solution.