[deathemperor]

well, this is very easy via using template conditions:
in showthread template: search for:

PHP Code:

</div>

<br />

$quickreply

<!-- currently active users -->
<if condition="$show['activeusers']"> 


around $quickreply, insert this:

PHP Code:

<if condition="is_member_of($post, specific id here)">$quickreply</if> 


insert the usergroups' id that you want to let them use the quick reply box. That should be all.
Thanks Xenon for guiding me this. It's really useful, isn't it ?