All records in QBIS can be secured in the following way, be they projects, tasks, contact records, etc. To secure a record you must click on the padlock icon if it is showing.
It is possible to set a QBIS record so that everyone can see it, only you can see it, or only selected people or roles within the company can see it. Advanced settings mean that certain individuals can be given access, or denied access, over and beyond their group settings.
When changing security for a project, then security is also changed for all the tasks and sub projects within that project.
QBIS has a very comprehensive and flexible security model. It is even possible to deny access to any record unless security has been applied!