[Logician]

edit showthread.php, find:

PHP Code:

if (!$getperms['canviewothers'] and $thread['postuserid']!=$bbuserinfo['userid']) {
  show_nopermission();
} 


After that add:

PHP Code:

  if ($forum['forumid']==X AND $bbuserinfo['usergroupid']==Y)
{
eval("standarderror(\"".gettemplate("error_youdonthavepermission")."\");");
exit;
} 


(Replace X with the forum id and Y with user group id)

Create a new template named "error_youdonthavepermission" and specify your custom message for no permission..

(Dont forget to enable their permissions in Admin CP)

If you want to apply the hack to 2 usergroups, change:

PHP Code:

    if ($forum['forumid']==X AND $bbuserinfo['usergroupid']==Y) 


as

PHP Code:

    if ($forum['forumid']==X AND ($bbuserinfo['usergroupid']==Y OR $bbuserinfo['usergroupid']==Z)) 


Enjoy..