How to remove contact from group using CiviCRM Entity and Drupal Rules
Just added a rules action "Unassign contact from group" to the dev release of CiviCRM 2 days ago, along with "Assign contact to group"

This will be in beta7 when it gets released in a couple of weeks, after some new PRs get tested and merged in.

If you have the user object you can use the "Load or Create CiviCRM Contact" action, then use the "Assign Contact to Group" action, to assign the contact one or more groups

This issue had some basic instructions:
https://www.drupal.org/node/2854256

Get the dev version here: https://www.drupal.org/project/civicrm_entity/releases/7.x-2.x-dev

or beta7 or above when it comes out...