To reproduce:
        public Form1()
        {
            InitializeComponent();
            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("ParentId", typeof(int));

            for (int i = 0; i < 3; i++)
            {
                dt.Rows.Add(i, "Parent" + i, -1);
            }
            Random rand = new Random();
            for (int i = 3; i < 15; i++)
            {
                dt.Rows.Add(i,"Child"+i,rand.Next(0,3));
            }

            this.radGridView1.Relations.AddSelfReference(this.radGridView1.MasterTemplate, "Id", "ParentId");
            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.EnableGrouping = true;
            this.radGridView1.EnableCustomGrouping = true;
            this.radGridView1.CustomGrouping+=radGridView1_CustomGrouping;
         }

private void radGridView1_CustomGrouping(object sender, GridViewCustomGroupingEventArgs e)
{
     
}