Extends the WP Statistics Plugin to show how many users are currently online.

functions.php

<?php

/*
  This gets added to your wordpress themes function.php or as a seperate plugin file
  
  This function extends the WP Statistics Plugin to show how many users are currently online
  And links it to the WP Statistics Page.
  
  The css class is added for you to style according to your needs.
  
  Will add future items later.
*/

// adds the link to the WP Toolbar
function custom_toolbar_link($wp_admin_bar) {
	
	// To see more info on user levels https://codex.wordpress.org/Roles_and_Capabilities#User_Level_to_Role_Conversion
	if( current_user_can( 'level_3' ) ){
		
	    $args = array(
	        'id' => 'wpstats',
	        'title' => '<span class="ab-icon dashicons-before dashicons-admin-users"></span><span class="ab-label">'. do_shortcode( "[wpstatistics stat=usersonline]" ) .' Users Currently Online</span>',
			'href' => '/wp-admin/admin.php?page=wps_online_page', 
		        'meta' => array(
		            'class' => 'wpstatsOnline', 
		            'title' => 'See users that are online right now.'
		            )
		
	    );
		$wp_admin_bar->add_node($args);
	}
}
add_action('admin_bar_menu', 'custom_toolbar_link', 999);

This could also be expanded with an admin jquery file to update the count every x seconds. But, I will leave that up to you.