Daniel Westermann-Clark

By default, WordPress network usernames cannot contain anything but lowercase letters and numbers. This plugin adds network options to let you include hyphens, underscores, or uppercase letters, if desired.
Furthermore, this plugin gives you the option to allow email addresses as usernames, or to allow all-numeric usernames (e.g. “1234”).
Finally, this plugin lets you override the minimum length for usernames (which defaults to four characters).
To follow updates to this plugin, visit:
https://danieltwc.com/
For help with this version, visit:
https://danieltwc.com/2011/network-username-restrictions-override-1-0/

Network Username Restrictions Override

Does what it says!

Override restrictions on WordPress network usernames.

Konstantin Obenland

This plugin lets you as a network administrator add users to your sites without having to remember or look up the user’s login name. It replaces the input field with a comfortable dropdown menu, so you can choose between all existing users that are not yet added to the site.
Don’t worry about code overhead or performance issues, the plugin will only be loaded when a network admin screen is requested.

WPMU Network Site Users Dropdown

Replace the input field for adding existing users to a site with a more comfortable dropdown menu!

Network Username Restrictions Override vs WPMU Network Site Users Dropdown