Source

File: wp-includes/ms-functions.php

function add_existing_user_to_blog( $details = false ) {
	if ( is_array( $details ) ) {
		$blog_id = get_current_blog_id();
		$result  = add_user_to_blog( $blog_id, $details['user_id'], $details['role'] );

		/**
		 * Fires immediately after an existing user is added to a site.
		 *
		 * @since MU (3.0.0)
		 *
		 * @param int           $user_id User ID.
		 * @param true|WP_Error $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;
	}
}