Invisible players can not be invited to the party. Thx DS

L2_GameServer/java/net/sf/l2j/gameserver/network/clientpackets/RequestJoinParty.java

@@ -61,6 +61,12 @@ public final class RequestJoinParty extends L2GameClientPacket
 		    return;
 
         if (target == null)
+        {
+            requestor.sendPacket(new SystemMessage(SystemMessageId.TARGET_IS_INCORRECT));
+            return;
+        }
+        
+        if (target.getAppearance().getInvisible())
         {
             requestor.sendPacket(new SystemMessage(SystemMessageId.TARGET_IS_INCORRECT));
             return;