//group grid's Collection for Country property
grid.CollectionView.GroupDescriptions.Add(new PropertyGroupDescription("Country"));

//group grid's Collection for Country property
grid.DataCollection.GroupAsync("Country");

public partial class MainWindow : Window
{
    
    public MainWindow()
    {
        InitializeComponent();

        //Initialize myList
        var myList = Customer.GetCustomerList(12);

        //set grid's itemsSource to myList
        grid.ItemsSource = myList;

        //group grid's Collection for Country property
        grid.DataCollection.GroupAsync("Country");

        //handling OptionsLoading event for 
        foreach (var col in grid.Columns)
            col.OptionsLoading += Col_OptionsLoading;
    }

    private void Col_OptionsLoading(object? sender, C1.WPF.Grid.GridColumnOptionsLoadingEventArgs e)
    {
        //Remove default options
        e.Menu.Items.Clear();

        var menuItem = new C1MenuItem();
        menuItem.Header = "Group By : " + e.Column.ColumnName;
        menuItem.Click += (s, oe) =>
        {
            //group by column
            grid.DataCollection.GroupAsync(e.Column.ColumnName);
            //close the menu
            e.Close();
        };
        e.Menu.Items.Add(menuItem);
    }
}