Not quite, since they would still be in the normal user group which has access, so they would have to be removed from that as well.
You can find the
Auto Group Change 1.1 beta mod here.
Just install that hack, and after that, find these lines in addpost.php:
<br /> if ( ($Totalposts == $posts) && !strstr($CurrGroups, $thegroup) ) {<br /> $NewGroups .= "$thegroup-";<br /> break;<br /> }<br />And change them into this:
<br /> if ( ($Totalposts == $posts) && !strstr($CurrGroups, $thegroup) ) {<br /> $NewGroups = str_replace("-3-", "-$thegroup-", $NewGroups);<br /> break;<br /> }<br />This should change users from the normal users group (with id 3) to the group you have set. It will only work with one level though, if you want them to be able to upgrade after that you will have to change it some more.
Haven't tried the code so it might not work, make backups of everything and some more after that before you try this. =]