What makes a CMS useful is the ability to administer content. Drupal has a built in system for user logins and granting permissions to different roles.
Users
Drupal can have as many site users as you need.
They can register and fill out a user profile with custom fields, which can be made public or private.
Log into a Drupal site by visiting /user.
There often a login link, provided by the secondary menu in the header.
Roles
Users can be assigned roles, which have a collection of permissions assigned.
Administrator
When you install Drupal, an administrator user is created that is assigned all permissions.
An Administrator role is also created that will automatically have all permissions assigned.
Authenticated
Any user that is logged in is Authenticated. Permissions can be assigned to this role for all logged in users.
Anonymous
Users who are not logged in are Anonymous. You can still grant or remove permissions from them.
Permissions
Drupal has many permissions for things like viewing unpublished content or accessing the admin theme. These can be assigned to different roles to grant users access to them.
Modules define permissions, so make sure to check after adding.