This feature is available in Enterprise and Ultimate editions.
From the main menu, select Product Planning > Backlog Groups.
Click the Add Backlog Groups button in the top right corner of the grid.
Enter the required information, and then click Save.
If you want to create a child Backlog Group underneath an existing Backlog Group, simply drag the Backlog Group onto a parent Backlog Group in the grid. Note that parent Backlog Groups display totals that include all of their backlog and the backlog of their children.