WordPress MultiSite: Display a List of Network User Emails by Role

style-admin.css

.emails {
	margin-bottom: 40px;
}

whos-what.php

<?php
/*
Plugin Name: Who's What?
Plugin URI: http://thecookingdish.com
Description: Displays a list of user emails by role under the 'Users' admin menu. Allows for bulk send via user's default email client.
Version: 1.1.0
Author: Chris Mower
Author URI: http://thecookingdish.com
*/

/**
 * If this file is called directly, abort
 */
if ( !defined( 'ABSPATH' ) ) {
	wp_die( __( "You are not allowed to access this page directly.", 'whoswhat' ) );
}

/**
 * Load admin styles
 */
function whoswhat_load_admin_styles() {
	wp_register_style( 'whoswhat_admin_styles',  plugin_dir_url( __FILE__ ) . '/style-admin.css' );
	wp_enqueue_style( 'whoswhat_admin_styles' );
}
add_action( 'admin_enqueue_scripts', 'whoswhat_load_admin_styles' );

/**
 * Create a page on the Admin screen under users.php
 */
function whoswhat_admin_plugin_menu() {
	add_submenu_page( 'users.php', 'Email Users by Role', 'Email Users', 'manage_network', 'email-users-by-role', 'whoswhat_display_the_emails');
}
add_action( 'admin_menu', 'whoswhat_admin_plugin_menu' );  // add to blog admin menus
add_action( 'network_admin_menu', 'whoswhat_admin_plugin_menu' ); // add to network admin menu

/**
 * Loop through each blog and grab the users
 *
 * @author Chris Mower
 * @since 1.0.0
 * @param string, $user_role, A pre-defined WordPress user role
 * @return array, a list of users as defined by $user_role
 */
function whoswhat_get_the_blog_users_emails( $user_role, $sendmail = false ) {

	// define global variable so that we can query the DB
	global $wpdb;

	// get a list of all the blogs on MultiSite install
	$blogs = $wpdb->get_results( "SELECT blog_id,domain,path FROM " . $wpdb->base_prefix . "blogs" );

	// define initial output
	$output = '';

	// define users array
	$users = array();

	// loop through each blog on the network
	foreach( $blogs as $blog ) {

		// get users from each of the blogs using the blog_id
		$blogusers = get_users( array(
			'blog_id' => $blog->blog_id,
			'role'    => $user_role,
			'orderby' => 'email',
		) );

		// loop through each of the returned users
		foreach( $blogusers as $bloguser ) {

			// check to see if they're not already in the users array
			if( !in_array( $bloguser, $users ) ) {

				// add to array based on user email
				$users[$bloguser->user_email] = $bloguser;
			}

		}

	}

	// sort the users array alphabetically
	ksort($users);

	// loop through the users array
	foreach ( $users as $user ) {

		// determine if the emails are for display or bulk email
		switch ( $sendmail ) {
			// for bulk email
			case true:
				$output .= esc_html( $user->user_email ) . ',';
				break;
			// for display
			case false:
			default:
				$output .= '<span><a href="mailto:' . esc_html( $user->user_email ) . '">' . esc_html( $user->user_email ) . '</a></span>, ';
				break;
		}

	}

	// return the output.
	return $output;
}

/**
 * Determine how to display the returned emails
 *
 * @author Chris Mower
 * @since 1.0.0
 */
function whoswhat_display_the_emails() {
	?>
	<div class="wrap">
		<h1>Network User Emails</h1>

		<div class="emails admin-emails">
			<h2>Administrator Emails</h2>
			<p><?php echo whoswhat_get_the_blog_users_emails( 'administrator' ); ?></p>
			<p>
				<a class="button" href="mailto:<?php echo whoswhat_get_the_blog_users_emails( 'administrator', true ); ?>">Email Administrators</a>
			</p>
		</div>

		<div class="emails editor-emails">
			<h2>Editor Emails</h2>
			<p><?php echo whoswhat_get_the_blog_users_emails( 'editor' ); ?></p>
			<p>
				<a class="button" href="mailto:<?php echo whoswhat_get_the_blog_users_emails( 'editor', true ); ?>">Email Editors</a>
			</p>
		</div>

		<div class="emails author-emails">
			<h2>Author Emails</h2>
			<p><?php echo whoswhat_get_the_blog_users_emails( 'author' ); ?></p>
			<p>
				<a class="button" href="mailto:<?php echo whoswhat_get_the_blog_users_emails( 'author', true ); ?>">Email Authors</a>
			</p>
		</div>

		<div class="emails contributor-emails">
			<h2>Contributor Emails</h2>
			<p><?php echo whoswhat_get_the_blog_users_emails( 'contributor' ); ?></p>
			<p>
				<a class="button" href="mailto:<?php echo whoswhat_get_the_blog_users_emails( 'contributor', true ); ?>">Email Contributors</a>
			</p>
		</div>

		<div class="emails subscriber-emails">
			<h2>Subscriber Emails</h2>
			<p><?php echo whoswhat_get_the_blog_users_emails( 'subscriber' ); ?></p>
			<p>
				<a class="button" href="mailto:<?php echo whoswhat_get_the_blog_users_emails( 'subscriber', true ); ?>">Email Subscribers</a>
			</p>
		</div>

		<div class="emails event-contributor-emails">
			<h2>Event Contributor Emails</h2>
			<p><?php echo whoswhat_get_the_blog_users_emails( 'ai1ec_event_assistant' ); ?></p>
			<p>
				<a class="button" href="mailto:<?php echo whoswhat_get_the_blog_users_emails( 'ai1ec_event_assistant', true ); ?>">Email Event Contributors</a>
			</p>
		</div>
	</div>
	<?php
}

Version 1.0.0

• Sorry about the crazy tab sizes. GitHub keeps auto-setting them to Tabs: 8. facepalm
• Displays Admins, Editors, Authors, Contributors, Subscribers, and a custom role called Event Contributers
• Feel free to fork and change it up.

Version 1.1.0

• Added alphabetical sorting, ascending order