Drupal Gardens offers several options for user registration and approval. You can have an invitation-only site; one where anyone can apply to join, but needs your approval to participate; and even a site where anyone with a Drupal Gardens user account can join your site instantly without any fuss or waiting.
Centralized user accounts
Drupal Gardens uses a centralized, OpenID-based system to manage user accounts. This means once you have a user account and username for site creation in Drupal Gardens or if you have created an account to join any Drupal Gardens website, you can use the same Drupal Gardens account to join any other Drupal Gardens site and to create as many Drupal Gardens websites as you like using the same user name.