As an example of replacing a built in group and then customizing it, we will hide the original
"Clipboard" group on the Home tab and build a new one without the Format Painter control.
In addition we will add the Mail Recipient button that Microsoft has hidden.
The Mail Recipient button sends a selection or worksheet to the body of an email.

You can add a built-in control like we do here or add a custom button that runs a macro.
On my Ribbon page you can find examples of how you can add a custom button.
http://www.rondebruin.nl/ribbon.htm


Excel’s built-in Clipboard group before we replace it with ours:



First we create a new workbook and save it with the name test.xlsx and we close this workbook.
Then we open the file in the UI Editor and paste the RibbonX below in the Office 2007 Custom UI Part.

We enter the following six lines in the UI editor (start and end lines of the XML)

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
<ribbon> 
<tabs>

<!-- Set visible to false for the Clipboard group on the Home tab-->
<tab idMso="TabHome"> 
<group idMso="GroupClipboard" visible="false"/>	
</tab>


<!-- Point to the Built-in tab to the ribbon -->
<tab idMso="TabHome"> 

<!-- Add Clipboard group -->
<group id="DupClipboard" label="Clipboard" insertBeforeMso="GroupClipboard" > 
	<splitButton idMso="PasteMenu" size="large" />
	<button idMso="Cut"/> 
	<button idMso="Copy"/> 
	<toggleButton idMso="SendCopySendToMailRecipient" />

	<dialogBoxLauncher>
	<button  idMso="ShowClipboard" />           
	</dialogBoxLauncher>
</group> 

</tab>


</tabs> 
</ribbon> 
</customUI>

Now we click the Save button in the Custom UI Editor and open the file test.xlsx in Excel.
If you have done it correctly, the Format Painter button will be replaced by the Mail Recipient like this: