How can I create a user automatically when someone submits a CiviCRM-Webform?
Backdrop
The simplest way would be to use the Webform Registration module, which has been tested to ensure it works with CiviCRM email fields.
Drupal
This conversation came up on the CiviCRM chat. I have done this for a site previously -- following instructions from @petednz's blog post here. (Note that blog post also has some exported Rules that you can download to start with and maybe save some time).
But if you want to start from scratch, here's what you need:

Webform CiviCRM
Rules
CiviCRM Entity



Create a tag within CiviCRM, something like &quot;Create Drupal User&quot;.
Set up the form, enable CiviCRM processing, and add a contact in the CiviCRM tab, customizing as you see fit.
On the &quot;Tags and Groups&quot; section for this contact, select the tag you created in #1. 
Go to the Rules page (Configuration &gt; Workflow &gt; Rules) and click &quot;Add New Rule&quot;
Name the rule and choose the React on Event as &quot;CiviCRM Entity Tag has been created&quot; 
As for Conditions, we first want to make sure there is no existing Drupal user for this contact, so we use the &quot;Drupal User Account exists for Contact&quot; and negate it (data selector value will be something like civicrm-entity-tag:entity-id-contact).
Then we do a data comparison on civicrm-entity-tag:entity-table to make sure that it is &quot;civicrm_contact&quot; (to be sure we are updating the right entity)
And as the final condition, another data comparison to make sure the tag that was created is the correct tag which should result in a Drupal user being created. Using the numeric tag ID from the tag you created in #1 (you can find this in the second column of the CiviCRM &quot;Manage tags&quot; section), we can compare to make sure that civicrm-entity-tag:tag-id equals this numeric ID. 
Finally, under Actions, we can use the &quot;Create Linked Drupal User Account&quot; action on the data selector civicrm-entity-tag:entity-id-contact. There are a few other options within here which you can set related to whether to activate/auto-sign in/send notification email as well.