Display Contacts from a Parent Account in a Child Account

function GetContacts() {

// get Contacts Sub grid
var accountChildContactsGrid = window.parent.document.getElementById("whatever your sub grid name is")


// Get the ID of the current account
var rootAccountID = Xrm.Page.data.entity.getId();

// Check that the subgrid is ready
if (accountChildContactsGrid == null){
    setTimeout('GetContacts()',1000);
    return;
}

// Construct FetchXML for contacts in this account and its child accounts
var fetchXml = "<fetch version='1.0' mapping='logical'>";
fetchXml += "<entity name='contact'>";
fetchXml += "<attribute name='fullname'/>";
fetchXml += "<attribute name='emailaddress1'/>";
fetchXml += "<order attribute='fullname' descending='false' />";
fetchXml += "<link-entity name='account' from='accountid' to='parentcustomerid' link-type='inner' >";
fetchXml += "<filter type='and'>";
fetchXml += "<condition attribute='accountid' operator='eq-or-under' value='" + rootAccountID + "' />";
fetchXml += "<condition attribute='accountid' operator='not-null' />";
fetchXml += "</filter>";
fetchXml += "</link-entity>";
fetchXml += "</entity>";
fetchXml += "</fetch>";

// make sure control is ready and set data and refresh the subgrid
if (accountChildContactsGrid.control != null){
    accountChildContactsGrid.control.SetParameter("fetchXml", fetchXml);
    accountChildContactsGrid.control.refresh();
}
else{
    setTimeout('GetContacts()',1000);
}