I have a drupal site with his own registration and a ios app. Now i want the app and the site to share users. I've been asked to expose the drupal registration as a web service. I proposed to use directly my drupal site registration putting it inside a web view but it doesn't seem to be enough. There is a module or something like this that does this? If not, how i can realize something like this? The drupal site is already active and full of users, so the best solution i can imagine is to make app users to register using the same procedure used for the site.
You'd want the Services module for this - with it, you can expose common Drupal tasks/functions as API calls, including registration/login.