You could use the Custom AggegrateFunction property. This way you could handle the aggegrate function and return the total value that will comply with your scenario. The event handler will be handled by CustomAggegrate server side event. Here is the snippet:

protected void WebGrid1_CustomAggregate(object sender, ISNet.WebUI.WebGrid.CustomAggregateArgs e)
{
    if (e.Column.Name == [ColumnName])
    {
        //Get the GroupTotal using custom function
        e.AggregateResult = GetTotal();
    }
}

Thanks.  I tried that, but I can't access all the rows for the group. When I code e.Rows in the CustomAggregate event stated above, it only brings back the rows on the current page.  I assume I'll have to go back to the dataset to perform my calcs instead of depending on the grid?  I can grab my grooupby value and use it to sum up my columns in the dataset? Is there an easier/cleaner way I'm  missing by using this customaggregate?

In your scenario, the calculation must be done on the datasource. The provided WebGrid information will not be sufficient for the scenario.