Currently there isn't a on/off switch to enable or disable account registration because there isn't a way to manually add account or invite users. Is a bummer, I now but sadly implement that is not that trivial but is considered for future releases. In the meanwhile, and if this system will be used only for private reasons, you can add an htpassword to the index.php file, this will cause the server to ask for a user/password combo (not the same as Chevereto user passwords) to enter the system.