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.

niksudan

This plugin allows administrators to create additional login details (username and password) for a registered user account. It restricts the additional logins from accessing their linked user’s profile page, as well as accessing the additional logins panel themselves.
The additional logins can be turned off at any time, preventing users from using the credentials to sign in. Descriptions can be given to logins that you create. Additional login usernames are treated like normal WordPress usernames, and cannot match an existing login. Likewise, new logins cannot have the same username as an additional login.
Useful for when wanting to have a few pairs of login details without giving away the main account’s password, as well as multiple team members, or just for convenience.
View it on GitHub: https://github.com/NikSudan/wp-additional-logins

Shared Users vs WordPress Additional Logins