View Schedule Group Headers Method

public void GroupHeaders(
	int top,
	int left,
	int bottom,
	int right,
	string caption
)

public static void CreateSingleCategoryScheduleWithGroupedColumnHeaders(Document doc)
{
    using (Transaction t = new Transaction(doc, "Create single-category with grouped column headers"))
    {
        // Build the schedule
        t.Start();
        ViewSchedule vs = ViewSchedule.CreateSchedule(doc, new ElementId(BuiltInCategory.OST_Windows));

        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.WINDOW_HEIGHT));
        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.WINDOW_WIDTH));
        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.ALL_MODEL_MARK));
        AddRegularFieldToSchedule(vs, new ElementId(BuiltInParameter.ALL_MODEL_COST));

        doc.Regenerate();

        // Group the headers in the body section using ViewSchedule methods
        vs.GroupHeaders(0, 0, 0, 1, "Size");
        vs.GroupHeaders(0, 2, 0, 3, "Other");
        vs.GroupHeaders(0, 0, 0, 3, "All");

        t.Commit();
    }
}

/// <summary>
/// Adds a single parameter field to the schedule
/// </summary>
public static void AddRegularFieldToSchedule(ViewSchedule schedule, ElementId paramId)
{
    ScheduleDefinition definition = schedule.Definition;

    // Find a matching SchedulableField
    SchedulableField schedulableField =
        definition.GetSchedulableFields().FirstOrDefault<SchedulableField>(sf => sf.ParameterId == paramId);

    if (schedulableField != null)
    {
        // Add the found field
        definition.AddField(schedulableField);
    }
}