Mailchimp

Use the Mailchimp List Subscribe plugin to quickly add a Mailchimp signup form block, widget, or shortcode to your WordPress site.
After installation, if you already have a Mailchimp account, you’ll log in with that account and then proceed to configure settings.
If you don’t have an account, you can create one directly in the plugin. After entering in all your personal details, you’ll need to activate your account via an email that will be sent to you. Once done, you’ll proceed to configure settings.
On the settings screen, you’ll select your Mailchimp list, choose merge fields and groups, and configure other options. Once done, you can now add the block, widget, or shortcode (<code>[mailchimpsf_form]</code>) to your site. Typically, installation and setup will take about 5-10 minutes, and absolutely everything can be done via the WordPress Setting GUI, with no file editing at all.
WordPress.com compatibility is limited to Business tier users only. <a href="https://mailchimp.com/help/ways-to-add-a-signup-form-in-wordpress/" rel="nofollow ugc">How to add a signup form if you have a WordPress.com site</a>.
<h3>Access Token Encryption</h3>
Starting in version 1.6.0, authentication has changed to use OAuth. As part of this process, we retrieve an access token that can be used to make API requests. To provide a high-level of security, this access token is encrypted before being stored in the WordPress database. In order to ensure this access token can be decrypted when used, the plugin relies on certain security constants that should remain unchanged.
With no additional configuration, we use the standard <code>LOGGED_IN_KEY</code> and <code>LOGGED_IN_SALT</code> constants that are normally set in your site’s <code>wp-config.php</code> file. Some sites make use of security plugins that rotate these constants on a periodic basis. When this happens, we won’t be able to decrypt the access token and you’ll need to reconnect your Mailchimp account to generate a new access token.
To prevent such issues, it is recommended to define two additional constants in your site’s <code>wp-config.php</code> file: <code>MAILCHIMP_SF_ENCRYPTION_KEY</code> and <code>MAILCHIMP_SF_ENCRYPTION_SALT</code>. These constants should consist of a combination of characters, preferably at least 32 characters long. Once set, these values should not be changed. For strong values, you can copy some of the values from <a href="https://api.wordpress.org/secret-key/1.1/salt/" rel="nofollow ugc">here</a> and use them. You’ll end up with additional code like the following in your <code>wp-config.php</code> file:
<pre><code>define( 'MAILCHIMP_SF_ENCRYPTION_KEY', 'put your unique phrase here' );
define( 'MAILCHIMP_SF_ENCRYPTION_SALT', 'put your unique phrase here' );
</code></pre>
If these constants are added after you’ve already authenticated with Mailchimp, you will need to reconnect your account. To avoid this, you can copy the values from <code>LOGGED_IN_KEY</code> and <code>LOGGED_IN_SALT</code> (if they exist) to <code>MAILCHIMP_SF_ENCRYPTION_KEY</code> and <code>MAILCHIMP_SF_ENCRYPTION_SALT</code> respectively.
<h3>Upgrading</h3>
If you are upgrading to version 1.2.1 and you used the widget in your sidebar previously, all you need to do is drag the <code>Mailchimp Widget</code> back into the sidebar, visit the Mailchimp settings page (which will have maintained your prior settings), click the “Update List” button, and you’re done!

Mailchimp List Subscribe Form

Add a Mailchimp signup form block, widget, or shortcode to your WordPress site.

Scott Reilly

This plugin allows you to restrict the usernames that new users may use when registering for your site.
If open registration is enabled for your site (via Settings -> General -> Membership (“Anyone can register”)), WordPress allows visitors to register for an account on your blog. By default, any username they choose is allowed so long as it isn’t an already existing account and it doesn’t include invalid (i.e. non-alphanumeric) characters.
Possible reasons for wanting to restrict certain usernames:
<ul>
<li>Prevent usernames that contain foul, offensive, or otherwise undesired words</li>
<li>Prevent squatting on usernames that you may want to use in the future (but don’t want to actually create the account for just yet) (essentially placing a hold on the username)</li>
<li>Prevent official-sounding usernames from being used (i.e. help, support, pr, info, sales)</li>
<li>Prevent official username syntax from being used (i.e. if all of your administrators use a prefix to identify themselves, you don’t want a visitor to use that prefix)</li>
<li>Prevent spaces from being used in a username (which WordPress allows by default)</li>
<li>Require that a username starts with, ends with, or contain one of a set of substrings (i.e. “support_”, “admin_”)</li>
<li>Require a minimum number of characters for usernames</li>
<li>Limit usernames to a maximum number of characters</li>
</ul>
When attempting to register with a restricted username, the visitor will be given an error notice that says: 
ERROR: This username is invalid. Please enter a valid username.
NOTE: This plugin does not put any restrictions on usernames that the admin chooses for users when creating user accounts from within the WordPress admin. This only restricts the names that users choose themselves when registering for your site.
SPECIAL NOTE: Many membership plugins implement their own user registration handling that often bypasses checks (and hooks) performed by WordPress. As such, it is unlikely that the plugin is compatible with them without special plugin-specific amendments.
Compatible with Multisite and BuddyPress as well.
Links: <a href="http://coffee2code.com/wp-plugins/restrict-usernames/" rel="nofollow ugc">Plugin Homepage</a> | <a href="https://wordpress.org/plugins/restrict-usernames/" rel="ugc">Plugin Directory Page</a> | <a href="https://github.com/coffee2code/restrict-usernames/" rel="nofollow ugc">GitHub</a> | <a href="http://coffee2code.com" rel="nofollow ugc">Author Homepage</a>
<h3>Hooks</h3>
The plugin exposes one filter for hooking. Typically, customizations utilizing this hook would be put into your active theme’s functions.php file, or used by another plugin.
c2c_restrict_usernames-validate (filter)
The ‘c2c_restrict_usernames-validate’ hook allows you to add your own customized checks for the username being registered. You can add additional restrictions or override the assessment performed by the plugin.
Arguments:
<ul>
<li>$valid (boolean): The assessment by the plugin about the validity of the username based on settings. True means username can be used.</li>
<li>$username (string): The username being registered.</li>
<li>$settings (array): The plugin’s settings.</li>
</ul>
Example:
<pre><code>/**
 * Add custom checks on usernames.
 *
 * Specifically, prevent use of usernames ending in numbers.
 *
 * @param bool $valid True if the username is valid, false if not.
 * @param string $username The username.
 * @param array $options Plugin options.
 */
function my_restrict_usernames_check( $valid, $username, $options ) {
 // Only do additional checking if the plugin has already performed its
 // checks and deemed the username valid.
 if ( $valid ) {
 // Don't allow usernames to end in numbers.
 if ( preg_match( '/[0-9]+$/', $username ) ) {
 $valid = false;
 }
 }
 return $valid;
}
add_filter( 'c2c_restrict_usernames-validate', 'my_restrict_usernames_check', 10, 3 );
</code></pre>

Restrict Usernames

Restrict the usernames that new users may use when registering for your site.

Mailchimp List Subscribe Form vs Restrict Usernames