permissions_from_file.module

<?php

use Drupal\user\Entity\Role;
use Drupal\user\Entity\User;

/**
 * Reads from file on shared drive and determines whether the specified user is in the list
 *
 * @param User $account: the user to test
 * @return bool: if the username is in the list or not
 */
function _permissions_from_file_is_in_file(User $account) {
  $contents = file_get_contents("/path/to/filename.csv");
  return ($contents) ? preg_match("/" . $account->getAccountName() . "/",$contents) : NULL;
}

/**
 * Adds or removes the role from the account
 * 
 * @param User $account: the user to add or remove the role from
 */
function _permissions_from_file_update_permissions(User $account) {
  $is_in_file = _permissions_from_file_is_in_file($account);
  $role = 'role_to_add';
  if ($is_in_file && !$account->hasRole($role)) {
    $account->addRole($role);
    $account->save();
  }
  else if ($is_in_file == FALSE && $account->hasRole($role)) {
    $account->removeRole($role);
    $account->save();
  }
  else if ($is_in_file == NULL) {
    //Failed to read file and should log an alert
    \Drupal::logger('permissions_from_file')->alert("Failed to read usernames file");
  }
}

/**
 * Implements hook_user_login
 * 
 * @param User $account: user who has just logged in
 */
function permissions_from_file_user_login(User $account) {
  _permissions_from_file_update_permissions($account);
}