This Expressionengine Extension will subscribe a user to a mailing list when logging in.
Simply add a form field called mailing_list
into your login form, and set its value to the mailing list id.
<input name="mailing_list" id="mailing_list" value="2" type="checkbox">
The extension will take care of adding the user to the mailing list.
Note : if the user is not logged in (wrong login or password), it will not be added to the list.