Make subgroups block visible only when group has a parent or children.

1. Go to /admin/build/block/configure/og_subgroups.
2. In Page specific visibility settings select 'Show if the following PHP code returns TRUE (PHP-mode, experts only).'
3. Paste the following code, that returns TRUE if the group has a parent or children.

    // Only print this block if the viewed group has a parent or children.
    if ($node = og_get_group_context()) {
      $gid = $node->nid;
      $top = og_subgroups_get_family($gid, 'up');
      $bottom = og_subgroups_get_family($gid, 'down');
      if ($top || $bottom) {
        return TRUE;
      }
    }