Yes, guest users can enter their email for receiving the out-of-stock notifications. Within the Magento 2 Out of Stock Notification extension, if the admin has disabled the module from the backend then the guest users will not be able to register for the out of stock notifications. After the module is enabled, the guest users can also enter their email id and hit the subscribe button to start receiving the out-of-stock notifications.
Please do contact us for any further queries or requirements. You can also check our magento2 Extensions for additional features.