property Group.BorderColor as Color
Specifies the color of group's border.

TypeDescription
Color A color expression that specifies the color of the group's list border. 
The BorderColor property changes the color for the group's list border. The BorderColor property has no effect if you have not specified the group's list background color using the BackColorList property.

The following sample shows how to show the border of the group's list:

With ExplorerTree1
    With .Groups.Add("Group")
        .BackColorList = vbWhite
        .BorderColor = vbBlack
        .Expanded = True
    End With
End With