d3 add multiple classes with function

You can assign multiple classes to elements by simply separating their names with spaces:

d3.selectAll(".user").attr("class", "user Michael");

But it seems that what you really need is to assign a data property to your elements for which it is much better to use the HTML5 data- attributes. So you could do:

d3.selectAll(".user").attr("data-name", function(d,i) { return "Michael #" + i; });

and later to obtain the name of a user:

d3.select(".user").attr("data-name")