If you can't change the json then you'll need to create a class to hold the data that the json represents;
public class MyData
{
public DataTable Table { get; set; }
}
then
MyData myObjectDT = JsonConvert.DeserializeObject<MyData>(myDynamicJSON);
grdJSON2Grid2.DataSource = MyData.Table;
grdJSON2Grid2.DataBind();
If you can change the json that is returned then get rid of the root "Table" property and make the json
[{"ID":1,"Username":"saaaaa","password":"pass1"},{"ID":2,"Username":"asdasdasd","password":"pass2"}]
the and code you already have should work.