The shared email module overrides the 'user' module's validation that prevents the same email address being used by more than one user. Works for both registration and account updates. Displays a warning to the user that they are using a shared email.
Based on http://drupal.org/node/15578#comment-249157
All this module does is modify the email address before it is validated by the user module.
Because it only changes the edit value rather than the form value, the validation will pass but the original unchanged email is still stored properly.