For this requirement I think you should use a Grouping's GroupHeaderTpl.
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
if (!X.IsAjaxRequest)
{
Store store = this.GridPanel1.GetStore();
store.DataSource = new object[]
{
new object[] { "group1", "1", "1" },
new object[] { "group1", "11", "11" },
new object[] { "group1", "111", "111" },
new object[] { "group2", "2", "2" },
new object[] { "group2", "22", "22" },
new object[] { "group2", "222", "222" }
};
}
}
</script>
<!DOCTYPE html>
<html>
<head runat="server">
<title>Ext.NET v2 Example</title>
<style>
.highlight-group {
background-color: red;
}
</style>
<script>
var isX = function (groupValue, records) {
var i,
isX = groupValue === "group2" ? true : false; // just for demonstration
for (i = 0; i < records.length; i++) {
// here you are getting access to each record of a current group
}
return isX;
};
</script>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:GridPanel ID="GridPanel1" runat="server">
<Store>
<ext:Store runat="server" GroupField="groupId">
<Model>
<ext:Model runat="server">
<Fields>
<ext:ModelField Name="groupId" />
<ext:ModelField Name="test1" />
<ext:ModelField Name="test2" />
</Fields>
</ext:Model>
</Model>
</ext:Store>
</Store>
<ColumnModel runat="server">
<Columns>
<ext:Column runat="server" Text="GroupId" DataIndex="groupId" />
<ext:Column runat="server" Text="Test1" DataIndex="test1" />
<ext:Column runat="server" Text="Test2" DataIndex="test2" />
</Columns>
</ColumnModel>
<Features>
<ext:Grouping runat="server">
<GroupHeaderTpl runat="server">
<Html>
<div <tpl if="this.isX(groupValue, children)" groupId === 'group2'">class="highlight-group"</tpl>>Group: {name}</div>
</Html>
<Functions>
<ext:JFunction Name="isX" Fn="isX" />
</Functions>
</GroupHeaderTpl>
</ext:Grouping>
</Features>
</ext:GridPanel>
</form>
</body>
</html>