Index: src/wp-includes/ms-functions.php
===================================================================
--- src/wp-includes/ms-functions.php	(revision 40293)
+++ src/wp-includes/ms-functions.php	(working copy)
@@ -415,7 +415,7 @@
 	$orig_username = $user_name;
 	$user_name = preg_replace( '/\s+/', '', sanitize_user( $user_name, true ) );
 
-	if ( $user_name != $orig_username || preg_match( '/[^a-z0-9]/', $user_name ) ) {
+	if ( $user_name != $orig_username || preg_match( '/[^a-z0-9 _.\-@]/', $user_name ) ) {
 		$errors->add( 'user_name', __( 'Usernames can only contain lowercase letters (a-z) and numbers.' ) );
 		$user_name = $orig_username;
 	}