Marc,
I got this figured out and hopefully this answer will help others. Its working with both the hello endpoint and members now. The fix was to NOT use a custom header with "api_key" or anything mentioned in my original post. The REST docs state using basic HTTP auth with the API key as the username. That still gave me an error in Zapier. What you need to do is have your HTTP auth field have the API key and a trailing pipe to format the request correctly, then it works. For example, my HTTP auth field is: "1234567890|" whereas with just the API key it was "1234567890". That trailing pipe made the magic happen!
