A: Yes, you can set anyone in your account as a "Viewer" and they would not be able to edit any of your files/collections, etc. and they would be counted as a user.

A user is simply anyone who you invite by email into your account.

However, you could use share links to any of your brands/collections/assets to anyone without them having to log in. It really just depends on the use case you are trying to serve :)