Coolite 

<GroupCommand OnEvent="OpenExistRack" Failure="Ext.MessageBox.alert('Loading Rack Window Failed', 'Error during the load of the rack details window.');">
                    <EventMask ShowMask="true" MinDelay="500" Msg="Loading Rack Details..." />
                        <ExtraParams>
                            <ext:Parameter Name="Values" Value="Ext.encode(#{MerchRackGrid}.getRowsValues())" Mode="Raw" />
                            <ext:Parameter Name="SelectedRecord" Value="Ext.encode(#{MerchRackGrid}.getRowsValues())" Mode="Raw" />
                            <ext:Parameter Name="SelectedGroup" Value="(#{groupId})" Mode="Raw" />
                        </ExtraParams>            
                </GroupCommand>

 <Listeners>
     <GroupCommand Handler="Ext.Msg.alert(command, 'Group id: '+groupId+'<br/>Count - ' + records.length);" />
 </Listeners>

<GroupCommand OnEvent="OpenExistRack" Failure="Ext.MessageBox.alert('Loading Rack Window Failed', 'Error during the load of the rack details window.');">
    <EventMask ShowMask="true" MinDelay="500" Msg="Loading Rack Details..." />
        <ExtraParams>
            <ext:Parameter Name="Values" Value="Ext.encode(#{MerchRackGrid}.getRowsValues())" Mode="Raw" />
            <ext:Parameter Name="SelectedRecord" Value="Ext.encode(#{MerchRackGrid}.getRowsValues())" Mode="Raw" />
            <ext:Parameter Name="SelectedGroup" Value="(#{merchgroupview}.getGroupId())" Mode="Raw" />
        </ExtraParams>           
</GroupCommand>

#{Store}.getGroupState();

#{merchgroupview}.lastGroupField;

<ext:Parameter Name="SelectedGroup" Value="groupId" Mode="Raw"></ext:Parameter>