Hi Dinesh,
Instead of setting the TableDescriptor.AllowNew property, try setting the ShowAddNewRecordBeforeDetails and ShowAddNewRecordAfterDetails property to false. And than You can use the AddNewRecord property of the GridTable to add new record in a grid. Please try the below code snippet and let me know if this helps.
this.gridGroupingControl1.Table.AddNewRecord.SetCurrent();
this.gridGroupingControl1.Table.AddNewRecord.BeginEdit();
this.gridGroupingControl1.Table.CurrentRecord.SetValue("ColumnName","Value");
this.gridGroupingControl1.Table.AddNewRecord.EndEdit();
Here is a sample.
GridAddNew.zipBest regards,
Haneef