Can you please share some more information on the specific setup you have? Do you want to password protect the backend or the front-end pages. There is and already implemented functionality in Sitefinity to password protect the backend (our login screen), if you have concerns about the security this provides, please take a look at the following
If you are concerned about users browsing your web site while it's still in production, I guess you implement an IP restriction in IIS, so that only a known range of IPs can access the project. Please inform me in detail if you need to use different functionality, so that I can assist you further.
the Telerik team