meloniq

Users who have not been assigned any Roles or Capabilities are called ‘orphans’. 
When using the <a href="https://web.archive.org/web/20160226091450/http://xentek.net/articles/528/implementing-the-wordpress-shared-users-table-trick/" rel="nofollow ugc">shared users table trick</a> to link up multiple WordPress installations, users who register on one of your blogs, are not given any privileges on the other blogs in the network. 
WP-Orphanage is a plugin that automatically adopts your orphan users by promoting them to the role of your choosing. 
By default it is the same as the default role set in the WP Options.
It does it in two ways:
<ol>
<li>Users who try to login to a different blog in the network than the one they signed up on, will be promoted at the time of login. The user won’t even know that it happened.</li>
<li>When the admin logs into the blog and views the users page, all orphan users – for that blog – are promoted automatically.</li>
</ol>
By taking a just in time approach, this plugin will not add any noticeable overhead to your WordPress blogs, while providing a seamless experience for the users and administrators.
This plugin is a extended version of WP-Orphanage plugin written by Eric Marden

WP-Orphanage Extended

Plugin to promote users with no roles set (the orphans) to the role from other blog where they registered or to default if any found.

Eric Marden

Users who have not been assigned any Roles or Capabilities are called ‘orphans’. When using the <a href="http://xentek.net/articles/528/implementing-the-wordpress-shared-users-table-trick/" rel="nofollow ugc">shared users table trick</a> to link up multiple WordPress installations, users who register on one of your blogs, are not given any privileges on the other blogs in the network. WP-Orphanage is a plugin that automatically adopts your orphan users by promoting them to the role of your choosing. By default it is the same as the default role set in the WP Options.
It does it in two ways:
<ol>
<li>Users who try to login to a different blog in the network than the one they signed up on, will be promoted at the time of login. The user won’t even know that it happened.</li>
<li>When the admin logs into the blog and views the users page, all orphan users – for that blog – are promoted automatically.</li>
</ol>
By taking a just in time approach, this plugin will not add any noticeable overhead to your WordPress blogs, while providing a seamless experience for the users and administrators.
<h3>License</h3>
The WP-Orphanage plugin was developed by Eric Marden, and is provided with out warranty under the GPLv2 License. More info and other plugins at: http://xentek.net
Copyright 2008 Eric Marden (email : wp@xentek.net)
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA

WP-Orphanage

Plugin to promote users with no roles set (the orphans) to the role of your choosing. Their role is upgraded when they login.

WP-Orphanage Extended vs WP-Orphanage