JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
<div id="groups">
    <div class="group-1">x</div>
    <div class="group-1">x</div>
    <div class="group-2">x</div>
    <div class="group-2">x</div>
    <div class="group-3">x</div>
</div>

CSS

#groups > div { border: solid 1px #ddd; padding: 5px; }

JavaScript

var groups = {};
$("#groups div").each(function(i) {
    var c = $(this).attr("class");
    if(!groups[c]) groups[c] = [];
    groups[c].push(this);
});
for(var i in groups) {
    $(groups[i]).wrapAll("<div id='" + i.replace('group', 'set') +"' />");
}