Daniel Miessler's directions are very good for setting up git to manage a website. However, they make the assumption that the user pushing the code is the same user that can publish websites. On shared hosting this is probably the case. On dedicated hosting it's probably not. If you're using OS X's built-in Apache server, it's definitely not. The user www has proper rights, and by default it isn't an ssh-able account. So you have to make some modifications to his directions.
- Create a group named git. This group contains all users that have permission to read and write to the git repo. In OS X, you can do this via Accounts in System Preferences.
- Add users to the group. In OS X, you can use dseditgroup to add non-visible users like www to the group.