Try something like this:
<xsl:for-each select="/docroot/users"/>
<xsl:variable name="u" select="."/>
<tr>
<xsl:for-each select="/docroot/columns/column">
<td>
<xsl:value-of select="$u/@*[name()=current()/@attributename]"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference