The syntax is UserHasBusinessUnit(['BU1','BU2','BU3']);

function UserHasBusinessUnit(businessUnits)
{
  var mybu = GetMyBusinessUnit();

  for (j = 0; j < businessUnits.length; j++)
  {
	// If there is a match, return true, found
	if (mybu == businessUnits[j]) return true;
  }  

  //otherwise return false
  return false;
}
function GetMyBusinessUnit() {
    var xml = "" +
	"<?xml version='1.0' encoding='utf-8'?>" +
	"<soap:Envelope xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'" +
	" xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'" +
	" xmlns:xsd='http://www.w3.org/2001/XMLSchema'>" +
	GenerateAuthenticationHeader() +
	"<soap:Body>" +
	"<Fetch xmlns='http://schemas.microsoft.com/crm/2007/WebServices'>" +
	"<fetchXml>" +
	" &lt;fetch mapping='logical' count='1'&gt;" +
	" &lt;entity name='businessunit'&gt;" +
	" &lt;attribute name='name' /&gt;" +
	" &lt;filter&gt;" +
	" &lt;condition attribute='businessunitid' operator='eq-businessid' /&gt;" +
	" &lt;/filter&gt;" +
	" &lt;/entity&gt;" +
	" &lt;/fetch&gt;" +
	"</fetchXml>" +
	"</Fetch>" +
	"</soap:Body>" +
	"</soap:Envelope>";

    var xmlHttpRequest = new ActiveXObject("Msxml2.XMLHTTP");
    xmlHttpRequest.Open("POST", "/mscrmservices/2007/CrmService.asmx", false);
    xmlHttpRequest.setRequestHeader("SOAPAction", "http://schemas.microsoft.com/crm/2007/WebServices/Fetch");
    xmlHttpRequest.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
    xmlHttpRequest.setRequestHeader("Content-Length", xml.length);
    xmlHttpRequest.send(xml);

    var resultXml = xmlHttpRequest.responseXML;

	var resultSet = resultXml.text;
    resultSet.replace('&lt;', '< ');
    resultSet.replace('&gt;', '>');

    var oXmlDoc = new ActiveXObject("Microsoft.XMLDOM");
    oXmlDoc.async = false;
    oXmlDoc.loadXML(resultSet);

	var result = oXmlDoc.getElementsByTagName('name');  

	return result[0].text;
}