How to embed username and password in a URL? We have a form that users need to fill out, but it is accessible only to authenticated users. We want to send a mass email to all users with an username and a password embedded in an URL, so that users can just click on that URL and log in to Drupal. The username and password will be common for all users. 

Is it possible to log in to Drupal through this way?
There is currently a sandbox module called URL Login waiting to get approved to become a full project. See the project page and the full project application issue.

That module provides a secure way to log in through an URL. If that module is useful to you, I suggest you mention that in the linked issue so that it can become a full project soon.