use this condition: <if condition="is_member_of($thread, 6)"> code goes here </if>
6 is the number of the usergroup. I don't know if this works, just a logical thought.
this is the line you must edit:
HTML Code:
<if condition="$show['guestuser']">
$thread[postusername]
<else />
<span style="cursor:pointer" onclick="window.open('member.php?$session[sessionurl]u=$thread[postuserid]')">$thread[postusername]</span>
</if>