You will find this setting at Dashboard -> Projects -> Settings -> Miscellaneous. You can set this feature enable/disable. User can make project public while creating project. Also, admin user set project privacy public/private in open project form.

Public projects can be visible to all registered users on site. If projects not assign to user, still he can access it. Tasks of public projects also visible to all registered users on site. If user not assign to public project’s task still he can access it.