We'll create fresh WordPress site with WPMU Network Site Users Dropdown installed. You have 20 minutes to test the plugin after that site we'll be deleted.
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.