Coolite 

<ext:Store runat="server" ID="store" RemoteGroup="true" RemotePaging="true" RemoteSort="true">
   <Proxy>
    <ext:HttpProxy Json="true" Method="POST" Url="TestService.asmx/GetData2">
    </ext:HttpProxy>
   </Proxy>
   <Reader>
    <ext:JsonReader Root="d.Data" IDProperty="Value" TotalProperty="d.TotalRecords">
     <Fields>
      <ext:RecordField Name="GroupItem" Mapping="GroupItem" />
      <ext:RecordField Name="Name" Mapping="Name" />
      <ext:RecordField Name="Value" Mapping="Value" />
      <ext:RecordField Name="TestDate" Mapping="TestDate" Type="Date" />
     </Fields>
    </ext:JsonReader>
   </Reader>
   <BaseParams>
    <ext:Parameter Name="start" Value="0"></ext:Parameter>
    <ext:Parameter Name="limit" Value="10">
    </ext:Parameter>
    <ext:Parameter Name="sort" Value="">
    </ext:Parameter>
    <ext:Parameter Name="dir" Value="">
    </ext:Parameter>
    <ext:Parameter Name="filters" Value="#{grid}.getFilterPlugin().buildQuery(#{grid}.getFilterPlugin().getFilterData())"
     Mode="Raw" Encode="true">
    </ext:Parameter>
    <ext:Parameter Name="groupBy" Value="">
    </ext:Parameter>
   </BaseParams>
   <Listeners>
    <LoadException Handler="var e = e || {message: response.responseText}; alert('Load failed: ' + e.message);" />
    <SaveException Handler="alert('save failed');" />
    <CommitDone Handler="alert('commit success');" />
    <CommitFailed Handler="alert('Commit failedReason: '+msg)" />
   </Listeners>
  </ext:Store>
  
  <ext:GridPanel runat="server" ID="grid" StoreID="store" Height="400">
   <ColumnModel ID="ColumnModel1" runat="server" IDMode="Inherit">
    <Columns>
     <ext:Column ColumnID="GroupItem" DataIndex="GroupItem" Header="GroupItem">
     </ext:Column>
     <ext:Column ColumnID="Value" DataIndex="Value" Header="Value">
     </ext:Column>
     <ext:Column ColumnID="Name" DataIndex="Name" Header="Name">
     </ext:Column>
     <ext:DateColumn ColumnID="TestDate" DataIndex="TestDate" Header="TestDate" Format="d/m/y">
     
     </ext:DateColumn>
    </Columns>
   </ColumnModel>
   <BottomBar>
    <ext:PagingToolbar runat="server" PageSize="10" PageIndex="1">
    </ext:PagingToolbar>
   </BottomBar>
   <View>
    <ext:GroupingView runat="server" ForceFit="true" HideGroupedColumn="false" StartCollapsed="true"
    EnableGrouping="true" EnableGroupingMenu="true"></ext:GroupingView>
   </View>
   <Plugins>
    <ext:GridFilters runat="server" ParamPrefix="filter">
     <Filters>
      <ext:StringFilter DataIndex="GroupItem" />
      <ext:NumericFilter DataIndex="Value" />
      <ext:StringFilter DataIndex="Name" />
     </Filters>
    </ext:GridFilters>
   </Plugins>
  </ext:GridPanel>

[WebMethod(EnableSession = true)]
  [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
  public SmartGridResults<DataItem> GetData2(int start, int limit,
   string sort, string dir,
   string filters, string groupBy)
  {
   //do some stuff
   // return data;
  }

<ext:Parameter Name="groupBy" Value="#{store}.groupField" Mode="Raw"></ext:Parameter>