Using the current 2.x-dev branch, I'm getting this error in the UI while the user is joining a group:

[Mailman Groups] Required function _mailman_groups_alter_og_confirm_subscribe() does not exist.

It appears both on the confirm page, and once the user clicks join.

A quick look at the code suggests the following function is to blame:

function mailman_groups_form_alter(&$form, $form_state, $form_id) {
  // Define an array of form_ids for forms we want to modify.
  $forms = array('user_profile_form', 'user_register', 'mailman_manager_admin_form', 'og_confirm_subscribe');
  // If this is not one of the forms we want to process, quick exit.
  if (!in_array($form_id, $forms)) {
    return;
  }

  // Call the matching form alter function.
  $alter = '_mailman_groups_alter_'. $form_id;
  if (_mailman_groups_checkfunc($alter)) {
    $alter($form, $form_state);
  }
}

There is no _mailman_groups_alter_og_confirm_subscribe function. Just a little alarming to the user, and seems unnecessary since it is not as though some external function was missing that the module required (since this appears to be something internal).