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use Drupal\group\Entity\GroupMembership;

// ...

public function submitForm(array &$form, FormStateInterface $form_state) {
  $csv_file_fid = $form_state->getValue('csv_upload');
  if (!empty($csv_file_fid)) {
    $file = File::load($csv_file_fid[0]);
    if ($file instanceof File) {
      $file_path = $file->getFileUri();
      $csv_data = array_map('str_getcsv', file($file_path));

      $header = [
        $this->t('Email'),
        $this->t('Username'),
        $this->t('Account Status'),
        $this->t('Group Membership'),
      ];
      $rows = [];

      foreach ($csv_data as $csv_row) {
        $email = $csv_row[0];

        $query = Database::getConnection()->select('users_field_data', 'u');
        $query->fields('u', ['uid', 'name']);
        $query->condition('u.mail', $email);
        $user = $query->execute()->fetchAssoc();

        if (!empty($user)) {
          // User found
          $username = $user['name'];
          $status = $this->t('Account Present');

          // Load the user's group memberships
          $group_memberships = GroupMembership::loadByUser($user['uid']);
          $group_membership_labels = [];
          foreach ($group_memberships as $group_membership) {
            $group = $group_membership->getGroup();
            $group_membership_labels[] = $group->label();
          }
          $group_membership_text = implode(', ', $group_membership_labels);
        } else {
          // User not found
          $username = '';
          $status = $this->t('Account Not Present');
          $group_membership_text = '';
        }

        $rows[] = [$email, $username, $status, $group_membership_text];
      }

      // Set the results in the form state
      $form_state->set('results', [
        'header' => $header,
        'rows' => $rows,
      ]);

      $form_state->setRebuild(); // Rebuild the form to display results
    }
  }
}