XSLT group select when not all nodes have the value

First of all I use xlst 1.0.

I have some structure:

<Users>
    <User name="Alex" surname="Jones"/>
    <User name="Mary" surname="Jones"/>
    <User name="Jane" surname="Wolf"/>
    <User name="John" />
</Users>

Now I need to make from this.

<Families>
    <Family surname="Jones">
         <Name>Alex</Name>
         <Name>Mary</Name>
    </Family>
    <Family surname="Wolf">
         <Name>Jane</Name>
    </Family>
    <Family surname="No surname">
         <Name>John</Name>
    </Family>
</Families>

When I use <xslt:key...> It only shows users that have surname.