For now, you'll need to create manufacturer profiles with username and password. However, they won't have access to the sellacious backend and manufacturer can log in the frontend only when they have the credentials.
Although, manufacturer login doesn't make sense and we'll take it as a suggestion and add it to our development roadmap and fix it in future releases of sellacious.
Username and passwords will be mandatory for any type of user. In sellacious, every user is a client and that's why username and passwords are mandatory. If you want it to be without the username/passwords, you can order the customization.