Posted this on saxon list yesterday. Not sure if my post was
successful.. so posting it here.. in case people in this list want to
test and check..

    <xsl:template match="Groups" mode="stream">
    <xsl:copy>
    <xsl:for-each-group select="Group/copy-of(.)"
    group-by="Group_Detail/GroupIdentifier">
    <xsl:variable name="GroupDetail" select="Group_Detail/copy-of(.)"/>
    <xsl:variable name="groupId" select="$GroupDetail/GroupIdentifier"/>
    <Formatted-Group groupId="{$groupId}">

    <xsl:copy-of select="current-group()/specifics"/>
    </Formatted-Group>
    </xsl:for-each-group>
    </xsl:copy>
    </xsl:template>