Index: src/wp-includes/ms-functions.php
===================================================================
--- src/wp-includes/ms-functions.php	(revision 41323)
+++ src/wp-includes/ms-functions.php	(working copy)
@@ -2110,17 +2110,16 @@
 		$blog_id = get_current_blog_id();
 		$result = add_user_to_blog( $blog_id, $details[ 'user_id' ], $details[ 'role' ] );
 
-		if ( ! is_wp_error( $result ) ) {
-			/**
-			 * Fires immediately after an existing user is added to a site.
-			 *
-			 * @since MU (3.0.0)
-			 *
-			 * @param int   $user_id User ID.
-			 * @param mixed $result  True on success or a WP_Error object if the user doesn't exist.
-			 */
-			do_action( 'added_existing_user', $details['user_id'], $result );
-		}
+		/**
+		 * Fires immediately after an existing user is added to a site.
+		 *
+		 * @since MU (3.0.0)
+		 *
+		 * @param int   $user_id User ID.
+		 * @param mixed $result  True on success or a WP_Error object if the user doesn't exist
+		 *                       or could not be added.
+		 */
+		do_action( 'added_existing_user', $details['user_id'], $result );
 
 		return $result;
 	}