In a usercontrol (.NET) you would normally do something like. 

if (HttpContext.Current.User.IsInRole("ActiveMembers"))
{
//dostuff here
}

Call probably fails because you haven't registered the xslt extension in xsltExtensions.config (well, it would help if you could tell us a bit more about the exact error message you receive)

Can't find the wiki reference atm, best I could find is this post on how to configure the extension

Hope this helps.

Regards,

/Dirk

Ok, sorry I was a little vague.

So here's specifically what i've done:

1. Compile the code from that post into a DLL, which all seems to go well. I did change the namespace from "bctllc.xsltext" to "xsltmembers"

2. Uploaded .dll into BIN folder

3. Update xsltExtensions.config to include:

    <ext assembly="/bin/xsltmembers" type="xsltmembers.members" alias="getxsltmembers" />

4. In my existing XSLT, added the following at the top:

    xmlns:xsltmembers="urn:getxsltmembers"

and added "xsltmembers" to the exlude portion...

5. I add this line, to get the logged in user's ID:

 <xsl:variable name="currentlogin" select="umbraco.library:GetCurrentMember()/@id"/><br/>

6. This all saves correctly, without errors... but then I try to add this code (from the original post):

<xsl:for-each select="xsltmembers:GetMemberGroupsForMember($currentlogin)/group">
        <xsl:value-of select="@id"/><xsl:value-of select="."/>
</xsl:for-each>

I get this error:

System.OverflowException: Value was either too large or too small for an Int32.
at System.Convert.ToInt32(Double value)
at System.Double.System.IConvertible.ToInt32(IFormatProvider provider)
at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
at System.Xml.Xsl.Runtime.XmlQueryRuntime.ChangeTypeXsltArgument(XmlQueryType xmlType, Object value, Type destinationType)
at System.Xml.Xsl.Runtime.XmlQueryContext.InvokeXsltLateBoundFunction(String name, String namespaceUri, IList`1[] args)
at (XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
at Root(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
at Execute(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlSequenceWriter results)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer, Boolean closeWriter)
at System.Xml.Xsl.XmlILCommand.Execute(IXPathNavigable contextDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter results)
at System.Xml.Xsl.XmlILCommand.Execute(IXPathNavigable contextDocument, XmlResolver dataSources, XsltArgumentList argumentList, TextWriter results)
at System.Xml.Xsl.XslCompiledTransform.Transform(IXPathNavigable input, XsltArgumentList arguments, TextWriter results)
at umbraco.presentation.webservices.codeEditorSave.SaveXslt(String fileName, String oldName, String fileContents, Boolean ignoreDebugging) in d:\TeamCity\buildAgent\work\7380c184e9fcd3ea\umbraco\presentation\umbraco\webservices\codeEditorSave.asmx.cs:line 124

Ok, what's weird is if I hard code in a member ID like this:

<xsl:for-each select="xsltmembers:GetMemberGroupsForMember(2780)/group">

It works fine...

So what's wrong with this?:

 <xsl:variable name="currentlogin" select="umbraco.library:GetCurrentMember()/@id"/>

is working on a reply...