Is it okay to ask prospective supervisor to recommend some article to read before joining his group for PhD?
Yes, definitely ask for some recommended reading! In fact, I wouldn't be surprised if your new supervisor suggests some papers to you even without prompting (mine did). As mentioned in the comments, supervisors are generally pleased when their students show an interest in the topic and want to learn more. You can look at some recent papers by your supervisor to get an idea of where to start, too.