A Sample CMB2 metabox with fields that show/hide based on other fields. In action: http://screencast.com/t/sXJ9rpOS

cmb2_sample.js

// Either create a new empty object, or work with the existing one.
window.JPry_CMB2 = window.JPry_CMB2 || {};

(function( window, document, $, app, undefined ) {
	'use strict';

	// Cache specific objects from the DOM so we don't look them up repeatedly.
	app.cache = function() {
		app.$ = {};
		app.$.select = $( document.getElementById( 'jpry_sample_selection' ) );
		app.$.field = $( document.getElementById( 'jpry_extra_field' ) );
		app.$.field_container = app.$.field.closest( '.cmb-row');
	};

	app.init = function() {
		// Store/cache our selectors
		app.cache();
		
		// Show the custom container when the selection is 'show-field'
		app.$.select.on( 'change', function( event ) {
			if ( 'show-field' === $(this).val() ) {
				app.$.field_container.show();
			} else {
				app.$.field_container.hide();
			}
		} ).trigger( 'change' );
	};

	$( document ).ready( app.init );
})( window, document, jQuery, JPry_CMB2 );

cmb2_sample.php

<?php
/**
 * Plugin Name: CMB2 Conditional Hide/Show example
 * Plugin URI: https://gist.github.com/JPry/184506fd72999b82e590
 * Description: Show an example of using JS to hide and show CMB2 fields based on other fields.
 * Version: 1.0.0
 * Author: Jeremy Pry
 * Author URI: http://jeremypry.com/
 * License: GPL2
 */

// Prevent direct access to this file
if ( ! defined( 'ABSPATH' ) ) {
	die( "You can't do anything by accessing this file directly." );
}

add_action( 'cmb2_admin_init', 'jpry_cmb2_init' );
/**
 * Register CMB2 metabbox and fields.
 */
function jpry_cmb2_init() {
	$cmb = new_cmb2_box( array(
		'id'           => 'jpry_sample_box',
		'title'        => __( 'JPry Sample Meta Box', 'jpry' ),
		'object_types' => array( 'post' ),
	) );

	$cmb->add_field( array(
		'name'    => __( 'Sample Selection Drop-Down', 'jpry' ),
		'id'      => 'jpry_sample_selection',
		'desc'    => __( 'This is a Sample Selection box. The item you select will show or hide the next field.', 'jpry' ),
		'type'    => 'select',
		'options' => array(
			'option-1'   => __( 'Option One', 'jpry' ),
			'option-2'   => __( 'Option Two', 'jpry' ),
			'show-field' => __( 'Show Extra Field', 'jpry' ),
		),
	) );

	$cmb->add_field( array(
		'name' => __( 'Extra Field', 'jpry' ),
		'id'   => 'jpry_extra_field',
		'desc' => __( 'This extra field will only be shown when the "Show Extra Field" option is selected above.', 'jpry' ),
		'type' => 'textarea',
	) );
}

add_action( 'admin_enqueue_scripts', 'jpry_enqueue_scripts' );
/**
 * Enqueue our script when needed.
 */
function jpry_enqueue_scripts() {
	$screen = get_current_screen();
	if ( ! isset( $screen->post_type ) || 'post' !== $screen->post_type ) {
		return;
	}

	$script_name = 'cmb2_sample.js';
	$url         = trailingslashit( plugin_dir_url( __FILE__ ) );
	wp_enqueue_script( 'jpry_cmb2_show_hide_example', $url . $script_name, array( 'jquery' ), '1.0.0' );
}

Hello JPry - well done, it works just fine!
...now I'm trying to apply this show/hide feature to a group rather than a single input element; I thought that this should work anyway since the js address to the trigger's nearest cmb-row, but don't - but I'm not a js ninja so surely I'm missing something...

Any help is more than welcome
Cheers
Paolo