Users have access to many areas of the platform. They can create/edit briefs, write content, and be granted access to the Research Cloud. Users can also access projects within the SEO Suite.
In contrast, writers can only access the Live Content Editor to view and make edits to the content they have been assigned. Most licenses come with a limited amount of user slots and a much larger amount of writer slots.
You can add a new Users and Writers to your License in Settings/ Users/ Invite New User or Invite Writer.
Have a look at: How to invite a user?