forms – Adding members to a group dynamically in Drupal 8

I am trying add members to a group programmatically using Groups module in Drupal 8, and I am doing the following to achieve this:

use DrupalCoreEntityContentEntityBase;
$roles = array('test_role');
$values = ('group_roles' => $roles, 'gid' => 1);
$account = $this->createUser();
$group = DrupalgroupEntityGroup::load(1);
$group->addMember($account, $values);

So, users are already present in the site. I want to add only those users to a group who have successfully paid for it. Hence, I am trying to write a custom solution that I add a user to an already existing group on the site. I came across this addMember($account, $values) function in the group module, however am still not sure on what values its arguments ($account and $values) take. Above piece of code does not add users to my group. Can someone please suggest what wrong I am doing here?