[kh99]

You could create a plugin using hook location showthread_start and this code:

$stu_usergroups = array(5, 6, 7); // usergroupis who will see "users browsing this thread"
$stu_forums = array(5); // forum id to show "users browsing this thread"

if (is_member_of($vbulletin->userinfo, $stu_usergroups) AND in_array($forumid, $stu_forums))
{
   $vbulletin->options['showthreadusers']=1; // 1=alphabetical all visitors, 
                                             // 2=random order all visitors, 
                                             // 3=alphabetical registered members only
                                             // 4=random order registered members only
}

where you'd have to edit the $stu_usergroups and $stu_forums arrays as needed, and you can also change the '1' to whichever type of display you want, as noted in the comments.

You could copy the entire block of code as many times as you needed (select it, copy it, and paste it at the end), if you want to allow different groups to see the display in different forums.