WP_User::add_role( string $role )

	public function add_role( $role ) {
		if ( empty( $role ) ) {
			return;
		}

		$this->caps[$role] = true;
		update_user_meta( $this->ID, $this->cap_key, $this->caps );
		$this->get_role_caps();
		$this->update_user_level_from_caps();

		/**
		 * Fires immediately after the user has been given a new role.
		 *
		 * @since WP-4.3.0
		 *
		 * @param int    $user_id The user ID.
		 * @param string $role    The new role.
		 */
		do_action( 'add_user_role', $this->ID, $role );
	}