JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://extjs.cachefly.net/ext-4.1.0-gpl/resources/css/ext-all.css">

CSS

.hide-sub-headers .x-group-sub-header {
    display: none;
}

JavaScript

Ext.create('Ext.data.Store', {
    storeId:'simpsonsStore',
    fields:['name', 'email', 'phone'],
    data:{'items':[
        { 'name': 'Lisa',  "email":"lisa@simpsons.com",  "phone":"555-111-1224"  },
        { 'name': 'Bart',  "email":"bart@simpsons.com",  "phone":"555-222-1234" },
        { 'name': 'Homer', "email":"home@simpsons.com",  "phone":"555-222-1244"  },
        { 'name': 'Marge', "email":"marge@simpsons.com", "phone":"555-222-1254"  }
    ]},
    proxy: {
        type: 'memory',
        reader: {
            type: 'json',
            root: 'items'
        }
    }
});

Ext.create('Ext.grid.Panel', {
    title: 'Simpsons',
    store: Ext.data.StoreManager.lookup('simpsonsStore'),
    columns: [
        { header: 'Name',  dataIndex: 'name'},
        { header: 'Group', cls: 'hide-sub-headers',
columns: [
            { header: 'Email', dataIndex: 'email'},
            { header: 'Phone', dataIndex: 'phone'}
        ]}        
    ],
    height: 200,
    width: 400,
    renderTo: Ext.getBody()
});