Sample data

Sample XML data that both examples utilize is as follows.

<?xml version="1.0" encoding="utf-8" ?>
<cars>
           <car manufacturer="Mazda" owner="Jason" />
           <car manufacturer="Opel" owner="Teemu" />
           <car manufacturer="Toyota" owner="Thomas" />
           <car manufacturer="Opel" owner="Henry" />
           <car manufacturer="Daimler" owner="Mika" />
           <car manufacturer="Toyota" owner="Colt" />
           <car manufacturer="Toyota" owner="Teemu" />
           <car manufacturer="Daimler" owner="Jason" />
</cars>

[Download sample data]

Selecting distinct nodes

How to select distinct manufacturers from the sample data? Well, like this for example:

//car[not(@manufacturer=preceding-sibling::car/@manufacturer)]/@manufacturer

How it works? It drops any car element that have same manufacturer attribute value that preceding sibling car element has (if such value exists in other words).

[Download XSLT stylesheet]

Grouping by manufacturers in XSLT stylesheet
In this scenario selecting distinct manufacturer nodes is the key thing, although logic is bit more complicated.  Distinct nodes are first selected into variable

<xsl:variable name="uniquemanufacturers" select="//car[not(@manufacturer=preceding-sibling::car/@manufacturer)]/@manufacturer" />

Then manufacturers are looped through using <xsl:for-each> and also in sorted order. For each manufacturer, car owners are queries as follows in the loop.

<xsl:variable name="owners" select="//car[@manufacturer=current()]/@owner" />

current() function refers to the current (context) node when for-each loop processing goes node by node. After this count of owners is printed out using

<xsl:value-of select="count($owners)" />

And finally car owners are listed in sorted order simply

<xsl:for-each select="$owners">
           <xsl:sort select="." />
           <xsl:value-of select="current()" /><br/>
</xsl:for-each>

[Download XSLT stylesheet]

Conclusion

I have demonstrated with simple examples how selecting distinct nodes and grouping can happen. You should be able to utilize this in your own stylesheets easily.

Related resources
Creating a Web.config Editor - Part 1 by Jason Gaylord
Creating a Web.config Editor - Part 2 by Jason Gaylord