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.

konnektiv

This Plugin allows administrators to restrict user login based on user roles. This is useful e.g. when you want anyone to register on your site but you do only want certain people (e.g. editors) to enter the logged in area.
Roles that are allowed to log in are set in a new menu entry in the Users menu.
This plugin was originally developed for the <a href="https://quality4digitallearning.org/" rel="nofollow ugc">globe – Community of Digital Learning</a> on behalf of <a href="https://www.giz.de/" rel="nofollow ugc">GIZ</a>.
<h4>Contact</h4>
<ul>
<li><a href="http://konnektiv.de/" rel="nofollow ugc">Konnektiv</a></li>
<li><a href="https://github.com/konnektiv/restrict-role-login" rel="nofollow ugc">Restrict Role Login on GitHub</a> – Report issues, contribute code</li>
</ul>

Restrict Role Login

Allows administrators to restrict user login based on user roles.

Shared Users vs Restrict Role Login