Muenchian grouping is the best approach; something like:
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl">
<xsl:output method="xml" indent="yes"/>
<xsl:key name="groups" match="/results/result" use="Store" />
<xsl:template match="/results">
<xsl:apply-templates select="result[generate-id() = generate-id(key('groups', Store)[1])]"/>
</xsl:template>
<xsl:template match="result">
<h1><xsl:value-of select="Store"/></h1>
<table id="{Store}">
<tr class="heading">
<th scope="col">Member Id</th>
<th scope="col">First Name</th>
<th scope="col">Last Name</th>
</tr>
<xsl:for-each select="key('groups', Store)">
<tr>
<td><xsl:value-of select="MemberLogin"/></td>
<td><xsl:value-of select="MemberFirstName"/></td>
<td><xsl:value-of select="MemberLastName"/></td>
</tr>
</xsl:for-each>
</table>
</xsl:template>
</xsl:stylesheet>
maybe easier to understand
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" >
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/results">
<xsl:apply-templates select="result[not(preceding-sibling::result/Store = Store)]/Store" />
</xsl:template>
<xsl:template match="result" >
<tr>
<td>
<xsl:value-of select="MemberLogin"/>
</td>
<td>
<xsl:value-of select="MemberFirstName"/>
</td>
<td>
<xsl:value-of select="MemberLastName"/>
</td>
</tr>
</xsl:template>
<xsl:template match="Store" >
<table id="{.}">
<tr class="heading">
<th scope="col">Member Id</th>
<th scope="col">First Name</th>
<th scope="col">Last Name</th>
</tr>
<xsl:variable name="temp" select="." />
<xsl:apply-templates select="//result[Store = current()]" />
</table>
</xsl:template>
</xsl:stylesheet>