lakrisgubben

This plugin uses BuddyPress’s notification system to let users subscribe to all new posts on a site (or many sites if you run Multisite). When a new post is published all users subscribed to the site gets a notification and optionally an email (can be turned off in their BuddyPress email settings).
<h4>Requires</h4>
<ul>
<li>BuddyPress 2.3 or later</li>
<li>WordPress 4.3 or later</li>
</ul>

BP Site Subscriber

This plugin uses BuddyPress's notification system to let users subscribe to all new posts on a site (or many sites if you run Multisite).

aliciagh

Easily search through all blogs into your WordPress Multisite by post title, post content or post author. 
Multisite Global Search doesn’t work with single WordPress installation and it must be activated for all sites using “network activate” in the Administration Panel. 
Currently in the following languages:
<ul>
<li>Dutch (nl_NL) by Niels Boos <a href="http://www.webdienstenzzp.nl" rel="nofollow ugc">Webdiensten ZZP</a></li>
<li>English</li>
<li>German (de_DE) by Dennis Vorpahl</li>
<li>French (fr_FR) by Fabrice Perrinel</li>
<li>Italian (it_IT) by Davide Tommasin</li>
<li>Norwegian Bokmal (nb_NO) by Tore Johnny Bråtveit</li>
<li>Portuguese (pt_PT) by Jose Anjos</li>
<li>Russian (ru_RU) by Sam Savinov</li>
<li>Romanian (ro_RO) by Alexander Ovsov <a href="http://webhostinggeeks.com/science/" rel="nofollow ugc">Web Hosting Geek Science</a></li>
<li>Serbian (sr_RS) by Andrijana Nikolic [Web Hosting Geeks] http://science.webhostinggeeks.com/multisite-global-search</li>
<li>Slovak (sk_SK) by Branco <a href="http://webhostinggeeks.com/blog/" rel="nofollow ugc">Web Hosting Geeks</a></li>
<li>Spanish (es_ES)</li>
</ul>
If you have created your own language pack, or have an update of an existing one, you can send <a href="https://codex.wordpress.org/Translating_WordPress" rel="nofollow ugc">gettext .po and .mo files</a> to me so that I can bundle it into Multisite Global Search.
** Features **
<ul>
<li>Multisite Global Search Widget. Show a search form in your sidebar.</li>
<li>Search across all network blogs or only in your blogs if you are logged.</li>
<li>Search on pages.</li>
<li>Administrative choice to perform searches by default on pages.</li>
<li>Administrative choice to disable search options.</li>
<li>Search results are showed in a page which contents the shortcode: <code>[multisite_search_result]</code></li>
<li>Show excerpted results <code>[multisite_search_result excerpt="yes"]</code></li>
<li>Entries on every site across your installation appear in search results immediately after publication.</li>
<li>Receive results from your complete blog network, even sites you do not own or control.</li>
<li>Customizable style sheet for widget and results page.</li>
<li>Two different form types, vertical and horizontal.</li>
<li>Put search form into your code with <code>Multisite_Global_Search::ms_global_search_vertical_form(your_results_page)</code> or <code>Multisite_Global_Search::ms_global_search_horizontal_form(your_results_page)</code></li>
<li>Insert search form in templates using the shortcode: <code>[multisite_search_form]</code>. <a href="http://grial.usal.es/agora/pfcgrial/multisite-search" rel="nofollow ugc">See the plugin page for more information</a>.</li>
</ul>
Last version contributed by <a href="https://github.com/lachele" rel="nofollow ugc">Lachele Foley</a>
<h3>Customization</h3>
You may make a few customizations to how the plugin looks and behaves. To make these customizations, you will need to be able to write to the multisite-global-search directory.
Adding Behavior Customizations
<ol>
<li>Make a file in the multisite-global-search directory called “config_local.php”.</li>
<li>See the file called “config.php” for the list of items you may customize. The file contains documentation for each of them.</li>
<li>Copy the lines containing variables you wish to change to config_local.php and alter them as desired. Take care to retain formatting, for example, the line-ending semicolon.</li>
<li>Make a backup copy of your config_local.php that is stored outside your WordPress installation!</li>
</ol>
Adding Look and Feel Customizations
<ol>
<li>Make a file in the multisite-global-search directory called “customstyle.css”.</li>
<li>Add your style alterations to this file. It will be loaded after the default style.css.</li>
<li>Make a backup copy of your customstyle.css that is stored outside your WordPress installation!</li>
</ol>

Multisite Global Search

Adds the ability to search through blogs into your WordPress Multisite installation.

BP Site Subscriber vs Multisite Global Search