By default, the IndentHeaderLeft property is 0. The IndentHeaderLeft property has effect only for group's caption, image and it does not affect the group's header background appearance. Use the BackColorGroup property to define a new background appearance using EBN files. Use the CP option ( copy option ) of the EBN files to define the way EBN file is arranged on the object. Use the IndentHeaderLeft property to indent the group's header from left side. Use the IndentHeaderRight property to indent the group's header from right side.  Use the IndentHeaderTop property to indent the group's header from top side. Use the IndentHeaderBottom property to indent the group's header from bottom side. Use the Indent property to indent the child items being displayed in the group's list. Use the IndentGroupLeft and  IndentGroupRight properties to specify the left and right indentation of the group's list.

The following screen shot shows the captions being displayed when indent properties has been used: 

The following screen shot shows the captions being displayed when indent properties has not been used: 

The following VFP sample changes the visual appearance for the groups:

with thisform.ExplorerTree1
	.BeginUpdate
	with .VisualAppearance
		.Add(1,"c:\images\group.ebn")
		.Add(2,"CP:1 0 0 -20 0")
	endwith
	.BackColorGroup = 0x2000000
	.GroupHeight = 48
	with .Groups
		with .Add("Group 1")
			.Alignment = 0
			.IndentHeaderLeft = 12
			.IndentHeaderTop = -8
		endwith
		with .Add("Group 2")
			.Alignment = 0
			.IndentHeaderLeft = 12
			.IndentHeaderTop = -8
		endwith
		with .Add("Group 3")
			.Alignment = 0
			.IndentHeaderLeft = 12
			.IndentHeaderTop = -8
		endwith
	endwith
	.EndUpdate
endwith

The following Delphi sample changes the visual appearance for the groups:

with AxExplorerTree1 do
begin
	BeginUpdate();
	with VisualAppearance do
	begin
		Add(1,'c:\images\group.ebn');
		Add(2,'CP:1 0 0 -20 0');
	end;
	(GetOcx() as EXPLORERTREELib.ExplorerTree).BackColorGroup := $2000000;
	GroupHeight := 48;
	with Groups do
	begin
		with Add('Group 1') do
		begin
			Alignment := EXPLORERTREELib.AlignmentEnum.LeftAlignment;
			IndentHeaderLeft := 12;
			IndentHeaderTop := -8;
		end;
		with Add('Group 2') do
		begin
			Alignment := EXPLORERTREELib.AlignmentEnum.LeftAlignment;
			IndentHeaderLeft := 12;
			IndentHeaderTop := -8;
		end;
		with Add('Group 3') do
		begin
			Alignment := EXPLORERTREELib.AlignmentEnum.LeftAlignment;
			IndentHeaderLeft := 12;
			IndentHeaderTop := -8;
		end;
	end;
	EndUpdate();
end

Here's the Template that generates the screens:

BeginUpdate
VisualAppearance
{
	Add(1,"c:/images/group.ebn")
	Add(2, "CP:1 0 0 -20 0")
}

BackColorGroup = 33554432
GroupHeight = 48

Groups
{
	"Group 1"
	{
		Alignment = 0
		IndentHeaderLeft = 12
		IndentHeaderTop = -8
	}
	"Group 2"
	{
		Alignment = 0
		IndentHeaderLeft = 12
		IndentHeaderTop = -8
	}
	"Group 3"
	{
		Alignment = 0
		IndentHeaderLeft = 12
		IndentHeaderTop = -8
	}
}
EndUpdate

You can find the EBN being used here.