This feature is available in all editions.
I have System Admin rights, but do not see a way to add a new top-level project. I can add release (child project) to my existing project, but how do I create a new higher level project?
All projects are created in the project tree as part of a single hierarchy under the root-level project, called System. You can control who can access the projects at any level of the project hierarchy. Typically, only one (or a few Administrators) have access to the System level.
Think of all the files and folders on your computer. The System project is the equivalent of your C: drive. You can create any structure below your C: drive to house your files. You can have a very flat structure or one that goes many levels deep. In either case, all your files are sitting on a single drive (ignoring, or course, a multi-drive server). In a similar way, you can create any structure you'd like in your project hierarchy under the System project: a flat list of projects or the root nodes of many (organizational, project, product owner, etc.) hierarchies below.
This means you are always extending the existing hierarchy instead of starting new root nodes, so you can create new projects by selecting Add Child Project under System or any other existing level.
Organization Hierarchy Example
In an organization with multiple projects defined, it is typically helpful to define a single organization level below the System node in the project hierarchy to encompass the list of projects. This allows a single point to which you can grant access to those who need visibility into multiple projects.
Most users won't see the entire hierarchy. A typical user may have access to a single project (e.g. just Project 1), multiple projects (e.g. Project 1 and Project 3), or the project hierarchy below Organization (i.e. Organization, Project 1, Project 2, and Project 3). Each member only knows about those projects to which they've been given access. To the member with access to only Project 1 and Project 3, the project hierarchy will look like a flat list.
Why have a System level?
The system is designed to allow inheritance along the project hierarchy. In other words, the member granted access to the Organization level above automatically gains that same level of access within each of the projects below. The System node gives the application a single point to use as the known top of the hierarchy.