There is a page password addon in the marketplace you can add that would place a password on any page (and optionally include all children). You would place that on your home page and give the password to those who you wanted to allow access. Like any shared password system, it is very weak security. You would be better enabling public registration and using the c5 core permission system to control access.

Another approach is http login. These are managed by the host account and web server. They sit outside of c5 and cannot be managed from within c5. Provision all depends on your server account and what facilities are installed.