If you have multiple WordPress installations using the same database, this plugin makes it possible for one 
installation to run off the user table of another installation. Main user table selection is completely configurable 
through the options panel, and permissions of the users can be different from blog to blog.
When you switch the user table from one blog to another, all level 10 users (administrators) of the blog you are 
switching to are also made level 10 users on the blog in which you installed the plugin. Only a level 10 user can 
make the switch.
After you switch, you should visit the Users section of the Admin panel to setup permissions for all other users. 
They will be listed as “No role for this blog” in WordPress 2.3, or with an empty Role column under “All Users” in WordPress 2.5/2.6.
Note that ideally the blog you are switching should not have more than the default admin user. Content created by all other users 
will automatically be assigned to more or less random users from the main blog when you switch (based on <code>user_id</code>). Future versions 
of the plugin may support migrating existing users and content.

Shared Users

Have your blog run off the user table of another blog, in the same database.

rownd

mhamann

Rownd is a user authentication and account platform that helps you convert more visitors to your website into actual (paying) customers.
We’re focused on passwordless sign-in that uses email and SMS as a means of distributing authentication links that allow users to verify 
their email/phone after they’ve had the opportunity to explore your product or service.
When you install the Rownd plugin for WordPress, you’ll bring our powerfully simple authentication and user account capabilities into 
your website, blog, or ecommerce site.
Rownd integrates with the existing WordPress user management system, which means we’re compatible with WooCommerce and many other 
WordPress addons! We’ll also detect whether you have WooCommerce installed and provide options to customize your customer experience 
during the checkout process.
Whether you’re accepting user signups already or want to start, Rownd makes it super-simple to get visitors and customers up and running quickly. 
Easily trigger our sign-in dialog by setting some HTML data attributes in your code or writing some simple Javascript and we’ll handle getting 
your users verified and signed in using our unique, passwordless approach.
To get started, you’ll need:
<ol>
<li>A free <a href="https://app.rownd.io" rel="nofollow ugc">Rownd account</a></li>
<li>A Rownd app key</li>
</ol>
Once you install this plugin, drop your app key and secret into our settings and you’ll be good to go!
Usage of this plugin is governed by Rownd’s <a href="https://rownd.io/terms-and-conditions" rel="nofollow ugc">terms of service</a>.

Rownd — Instant user accounts and authentication

Instantly turn visitors into users with Rownd's radically simple, user-centric authentication.

Shared Users vs Rownd — Instant user accounts and authentication