Populate html table on jQuery success event

Try this:

for (var i = 0; i < len; i++) {
   var username = data[i].username;
   var name  = data[i].name;
   var lastname = data[i].lastname;
   $('#table tbody').append('<tr><td>'+username+'</td><td>'+name+'</td><td>'+lastname+'</td></tr>')
}

You could try this:

    var table = $("#table tbody");
    $.each(data, function(idx, elem){
        table.append("<tr><td>"+elem.username+"</td><td>"+elem.name+"</td>   <td>"+elem.lastname+"</td></tr>");
    });

More information can be found here: http://api.jquery.com/jQuery.each/