You could add your friends as Users with their own username and password.
Then check out the is_user_logged_in conditional tag — http://codex.wordpress.org/Function_Reference/is_user_logged_in — and modify your theme so that the loop only runs/ the page only shows content if that conditional returns true.
I haven’t had much to do with the built-in Private Posts wordpress thing, but on a post-by-post basis you could set each post you publish to Visibilty: Private. Again, I haven’t played with this functionality but it would mean no plugins or theme modifications required.