The polite way to ask for a guest's identity is to simply ask them for their name. You could say something like, "Could you please tell me your name?" or "I'm sorry, I didn't catch your name. Could you tell me again?" It's important to be polite and respectful when asking for someone's name, as it is a personal piece of information that they may not want to share with everyone. It's also a good idea to provide a reason for why you are asking for their name, such as to properly greet them or to keep track of who is attending an event.