Collective guilt for registrants from the same IP address

If someone tries to register from an IP address that a silenced user
has registered from, prevent it.

When silencing someone, silence everyone else who registered from the
same IP address.

plugins/RegisterThrottle/Registration_ip.php

@@ -111,8 +111,33 @@ class Registration_ip extends Memcached_DataObject
      *
      * @return array magic three-false array that stops auto-incrementing.
      */
+
     function sequenceKey()
     {
         return array(false, false, false);
     }
+
+    /**
+     * Get the users who've registered with this ip address.
+     *
+     * @param Array $ipaddress IP address to check for
+     *
+     * @return Array IDs of users who registered with this address.
+     */
+
+    static function usersByIP($ipaddress)
+    {
+        $ids = array();
+
+        $ri            = new Registration_ip();
+        $ri->ipaddress = $ipaddress;
+
+        if ($ri->find()) {
+            while ($ri->fetch()) {
+                $ids[] = $ri->user_id;
+            }
+        }
+
+        return $ids;
+    }
 }