I think the Dashboard's Table widget hasn't had row grouping yet, not like the Table or DataTables one in KoolReport Pro. So far you would need to use KWidget in Dashboard to get row grouping:

class CoreTableDemo extends KWidget
{
    protected function dataSource()
    {
        ...
    }

    protected function onCreated()
    {
        $this
        ->use(\koolreport\widgets\koolphp\Table::class)
        ->settings([
            // set "columns", "options", "grouping" of Table widget here: https://www.koolreport.com/docs/koolphp/table/#row-group
        ]);
    }
} 

or:

class DataTablesDemo extends KWidget
{
    protected function dataSource()
    {
        ...
    }

    protected function onCreated()
    {
        $this
        ->use(\koolreport\datagrid\DataTables::class)
        ->settings([
            // set "columns", "options", "clientRowGroup" of DataTables widget here: https://www.koolreport.com/docs/datagrid/datatables/#clientrowgroup-(version-%3E=-5.0.0)
        ]);
    }
}