var wb = new XLWorkbook();
var ws = wb.Worksheets.Add("Outline");

ws.Outline.SummaryHLocation = XLOutlineSummaryHLocation.Right;
ws.Columns(2, 6).Group(); // Create an outline (level 1) for columns 2-6
ws.Columns(2, 4).Group(); // Create an outline (level 2) for columns 2-4
ws.Column(2).Ungroup(true); // Remove column 2 from all outlines

ws.Outline.SummaryVLocation = XLOutlineSummaryVLocation.Bottom;
ws.Rows(1, 5).Group(); // Create an outline (level 1) for rows 1-5
ws.Rows(1, 4).Group(); // Create an outline (level 2) for rows 1-4
ws.Rows(1, 4).Collapse(); // Collapse rows 1-4
ws.Rows(1, 2).Group(); // Create an outline (level 3) for rows 1-2
ws.Rows(1, 2).Ungroup(); // Ungroup rows 1-2 from their last outline

// You can also Collapse/Expand specific outline levels
//
// ws.CollapseRows(Int32 outlineLevel)
// ws.CollapseColumns(Int32 outlineLevel)
//
// ws.ExpandRows(Int32 outlineLevel)
// ws.ExpandColumns(Int32 outlineLevel)

// And you can also Collapse/Expand ALL outline levels in one shot
//
// ws.CollapseRows()
// ws.CollapseColumns()
//
// ws.ExpandRows()
// ws.ExpandColumns()

wb.SaveAs("Outline.xlsx");