I know there is a script to block users from seeing your site like this one

http://www.createblog.com/forums/index.php...=55042&hl=block

but is there one to only allow users on the list to see your site?

Xanga features a protective posting option. Go to your subscriptions list and pick out members you only want to see your xanga, and those are the only people who will see your posts.