Here you are also a new article addressing the same topic: http://docs.telerik.com/devtools/aspnet-ajax/controls/editor/functionality/toolbars/using-ribbonbar

The way to add tools into tabs and groups is similar to the plain approach to add tools programmatically—http://docs.telerik.com/devtools/aspnet-ajax/controls/editor/functionality/toolbars/buttons/add-standard-buttons#setting-tools-programmatically-in-the-code-behind. The difference is that you should make sure to use the EditorToolGroup instance's Tab and tag properties to define the tab and the group. 

Here you are an example for that: 

ASPX
<telerik:RadEditor runat="server" ID="RadEditor1" ToolbarMode="RibbonBar">
</telerik:RadEditor>


Codebehind
protected void Page_Load(object sender, System.EventArgs e)
{
    //===== Add new section group in Home tab =====
    EditorToolGroup myGroup = new EditorToolGroup();
    myGroup.Tab = "Home";
    // Define teh place of the new toolgroup via the Tab property.
    myGroup.Tag = "My Section";
    // And the group by using the Tag property.
 
    EditorTool bold = new EditorTool();
    bold.Name = "Bold";
    bold.ShortCut = "CTRL+B";
    myGroup.Tools.Add(bold);
 
    // Add a custom toolstrip
    EditorToolStrip toolStrip = new EditorToolStrip("myStrip");
    toolStrip.Tools.Add(new EditorTool("Bold"));
    toolStrip.Tools.Add(new EditorTool("Italic"));
    toolStrip.Tools.Add(new EditorTool("Underline"));
    myGroup.Tools.Add(toolStrip);
 
 
    RadEditor1.Tools.Add(myGroup);
    //===== End =====
 
    //===== Add new Tab in the toolbar =====
    EditorToolGroup mySecondGroup = new EditorToolGroup();
    // The new tab is declared by using the Tab property
    mySecondGroup.Tab = "New Tab";
    mySecondGroup.Tag = "My Section in Tab";
 
    EditorTool italic = new EditorTool();
    italic.Name = "Italic";
    italic.ShortCut = "CTRL+I";
    mySecondGroup.Tools.Add(italic);
 
    RadEditor1.Tools.Add(mySecondGroup);
    //===== End =====
}