If your goal is to be able to hide content from others, you may do so by setting the state of folders or other content items to "private."

However, if what you want to do is to make particular content available to some logged in users, but not others, it's a different issue. We normally set up site owners with a security profile that doesn't allow this. This isn't so much to restrict you as to hide some complexity. Ask the web team to elevate your security profile and you'll gain a "Sharing" tab that will allow you to set permissions.

This may not be quite as useful as you hope, as site owners have no way to add users. But, the web team may add a limited number of login ids that you may use for this purpose.