[buro9]

I think you missed out something like:

$threadinfo['privateusers'] = $threadinfo['privateusers'] ? implode(";",unserialize($threadinfo['privateusers'])) : "";

in the showthread.php file, and then change $restrictedusers to $threadinfo[privateusers] in the showthread template.

And I've made it admin only, by doing things like the following in the newthread and the edit thread templates:

<if condition="$foruminfo['allowprivate'] == 1 and is_member_of($bbuserinfo, 6)">

Where 6 is the id of the Admins usergroup