Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 26930)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -197,11 +197,17 @@
 	$return = array();
 
 	// Check the type of request
-	if ( isset( $_REQUEST['autocomplete_type'] ) )
+	if ( isset( $_REQUEST['autocomplete_type'] ) && in_array( $_REQUEST['autocomplete_type'], array( 'add', 'search' ) ) )
 		$type = $_REQUEST['autocomplete_type'];
 	else
 		$type = 'add';
 
+	// Check the desired field for value
+	if ( isset( $_REQUEST['autocomplete_value'] ) && in_array( $_REQUEST['autocomplete_value'], array( 'user_login', 'user_email' ) ) )
+		$value_field = $_REQUEST['autocomplete_value'];
+	else
+		$value_field = 'user_login';
+
 	// Exclude current users of this blog
 	if ( isset( $_REQUEST['site_id'] ) )
 		$id = absint( $_REQUEST['site_id'] );
@@ -223,7 +229,7 @@
 		$return[] = array(
 			/* translators: 1: user_login, 2: user_email */
 			'label' => sprintf( __( '%1$s (%2$s)' ), $user->user_login, $user->user_email ),
-			'value' => $user->user_login,
+			'value' => $user->$value_field,
 		);
 	}
 
Index: src/wp-admin/js/user-suggest.js
===================================================================
--- src/wp-admin/js/user-suggest.js	(revision 26930)
+++ src/wp-admin/js/user-suggest.js	(working copy)
@@ -8,17 +8,23 @@
 			position.my = 'right top';
 			position.at = 'right bottom';
 		}
-		$( '.wp-suggest-user' ).autocomplete({
-			source:    ajaxurl + '?action=autocomplete-user&autocomplete_type=add' + id,
-			delay:     500,
-			minLength: 2,
-			position:  position,
-			open: function() {
-				$( this ).addClass( 'open' );
-			},
-			close: function() {
-				$( this ).removeClass( 'open' );
-			}
+		$( '.wp-suggest-user' ).each( function(){
+			var $this = $( this ),
+				autocompleteType = ( typeof $this.data( 'autocompleteType' ) !== 'undefined' ) ? $this.data( 'autocompleteType' ) : 'add',
+				autocompleteValue = ( typeof $this.data( 'autocompleteValue' ) !== 'undefined' ) ? $this.data( 'autocompleteValue' ) : 'user_login';
+
+			$this.autocomplete({
+				source:    ajaxurl + '?action=autocomplete-user&autocomplete_type=' + autocompleteType + '&autocomplete_value=' + autocompleteValue + id,
+				delay:     500,
+				minLength: 2,
+				position:  position,
+				open: function() {
+					$( this ).addClass( 'open' );
+				},
+				close: function() {
+					$( this ).removeClass( 'open' );
+				}
+			});
 		});
 	});
-})( jQuery );
\ No newline at end of file
+})( jQuery );
Index: src/wp-admin/network/site-new.php
===================================================================
--- src/wp-admin/network/site-new.php	(revision 26930)
+++ src/wp-admin/network/site-new.php	(working copy)
@@ -106,6 +106,8 @@
 $title = __('Add New Site');
 $parent_file = 'sites.php';
 
+wp_enqueue_script( 'user-suggest' );
+
 require( ABSPATH . 'wp-admin/admin-header.php' );
 
 ?>
@@ -138,7 +140,7 @@
 		</tr>
 		<tr class="form-field form-required">
 			<th scope="row"><?php _e( 'Admin Email' ) ?></th>
-			<td><input name="blog[email]" type="text" class="regular-text" title="<?php esc_attr_e( 'Email' ) ?>"/></td>
+			<td><input name="blog[email]" type="text" class="regular-text wp-suggest-user" data-autocomplete-type="search" data-autocomplete-value="user_email" title="<?php esc_attr_e( 'Email' ) ?>"/></td>
 		</tr>
 		<tr class="form-field">
 			<td colspan="2"><?php _e( 'A new user will be created if the above email address is not in the database.' ) ?><br /><?php _e( 'The username and password will be mailed to this email address.' ) ?></td>