Help me set usergroupid to a custom user field

[Galen]

Take a look at the second-to-last line (DB_site).

// ###################### Start do moderate and coppa #######################
if ($HTTP_POST_VARS['action']=="domoderate") {

  while (list($key,$val)=each($validate)) {
    if ($val==1) {

      $user=$DB_site->query_first("SELECT username,email FROM user WHERE userid=$key");
      $username=unhtmlspecialchars($user[username]);

      eval("\$message = \"".ereg_replace("\"","\\\"",$email)."\";");
      eval("\$subject = \"".ereg_replace("\"","\\\"",$subject)."\";");

      mail ($user[email],$subject,$message,"From: $webmasteremail");

      $DB_site->query("UPDATE user SET usergroupid=2 WHERE userid=$key");
    }
  }

  echo "<p>Accounts validated and users notified</p>";

}

Where it says SET usergroupid=2, I would like instead for it to set the usergroupid to a custom user field number that is selected at registration.

So, instead of SET usergroupid=2, I'd like it to say, SET usergroupid=field8.

Except, this doesn't work - I don't know how to properly insert the value of field8 into usergroupid. Any suggestions?