When you hover the mouse cursor over a group this button appears. Click this button to isolate branches of the chart. The group you drilled down on appears at the top of the chart and all of its branch groups appear below it. After drilling down, "drill up" buttons appear.
The button on the left drills up one level
The button on the right drills all the way up to the top level of the chart
The white space is called the Resource Area. Right-click in this area to open a context menu.
Delete Selected Resource(s) - Deletes one or more selected resource assignments
Select ALL - Selects all resource assignments in the group prior to deleting them
Add Resource - Opens the Edit Resource Dialog to a new, empty, record
Edit Resource - Opens the Edit Resource Dialog with the current resource assignment loaded. Resources can also be assigned by dragging them from the Resources Table Flyout and you can also drag and drop a resource from one group to another.