Well, you could try creating a plugin using hook location forumdisplay_complete and code like this (change the X on the first line to the id of the forum where you want it to display):
if ($forumid == X)
{
if ($result = $vbulletin->db->query_read_slave("SELECT * FROM " . TABLE_PREFIX . "user WHERE usergroupid=20 ORDER BY username"))
{
while ($user = $vbulletin->db->fetch_array($result))
{
$members[] = $user;
}
$template = vB_Template::create('my_memberlist');
$template->register('members', $members);
$template_hook['forumdisplay_below_threadlist'] .= $template->render();
}
}
then create a new template something like this:
Members:<br /><br />
<vb:each from="members" key="key" value="member">
<a href="{vb:link member, {vb:raw member}}">{vb:var member.username}</a><br />
</vb:each>
<br/><br/>
In this example I named the new template 'my_memberlist', but you can name it anything you want as long as you change it in the plugin code as well.
Of course this is a simple example, html isn't really my thing so I'll leave it up to you to make it look good.