If you know which leaf-node organization owns the project or task, find its Organization view, which lists the people who have specific privileges in that organization. The details column lists the projects to which each privilege applies — either "All projects & tasks" or a list of specific projects and/or tasks. If the Organization view is very long, use the browser's Find feature (Ctrl-F or Command-F) to search for the project or task number.
People with "all projects" authority in the leaf-node organization, and people who have authority scoped to the parent organization, all have implicit authority for any project or task owned by the leaf-node organization.
