How do I assign a role to a user after the user's email is confirmed?

You'll want to take a look at Rules and LoginToboggan (as per this answer on Drupal Answers).

Rules will allow you to add a "trigger", such as adding a role to a user, and LoginToboggan will provide the "trigger" for when the account is validated.

You can use hook_user_login(&$edit, $account) which is called from user_login_finalize().

When $edit parameter is empty is because is called from user_pass_reset() menu callback:

// user_login_finalize() also updates the login timestamp of the // user, which invalidates further use of the one-time login link. user_login_finalize(); 

This example should work:

function MYMODULE_user_login(&$edit, $account) { if (empty($edit)) { // @todo Add role to $account->roles array } }