User accounts for customers
This is a setting that's built into the core of Drupal. On D7/D8 the Who can register accounts? (Public registrations in D6) setting adjusts who can create accounts on your Drupal installation. 


Administrators only
Visitors
Visitors, but administrator approval is required


The settings form can be found under /admin/config/people/accounts (/admin/user/settings in D6).