Zen Cart does not currently have this feature built-in.

However, it is possible to do it with custom code. Essentially you'd need to build a security model, decide "who" *is* allowed to see certain products/categories. Then set up a group to allow that access. You'd need to store the authorization information in the database. You'd need to build a validation checker to ensure someone doesn't access a prohibited item by typing in the product number or accessing it via a search. And then you'd need to validate it during the checkout process as well.