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AppThemes

With the current WordPress administration area, the only way to add existing users to a certain blog is by entering their usernames, one by one. It’s very time consuming.
With this plugin, you can do it in bulk, by using the familiar “Change role to…” dropdown:
<ol>
<li>Go to WP Admin -> Users.</li>
<li>Click the “Entire Network” link.</li>
<li>Check the users you want to add.</li>
<li>Choose the new role from the dropdown.</li>
<li>Press “Change”.</li>
</ol>
Note that you have to be a super-admin to be able to do this.

User Management Tools

Allows super-admins to quickly add users to a particular blog in a multisite installation.

User Management Tools vs UpStream: a Project Management Plugin for WordPress