How can you ask a girl to be your girlfriend when you are already dating?
you are the worlds most beautiful girl i ever seen, would you like to be my friend. ANSWER: are you crazy. you cant. you are going out with another girl. that is called cheating. also if you do that you will be labaled a cheater. So choose ONE. you cant have both.
please pay no attention to the part above this. If I am right you mean that you are dating a girl but she is not your girlfriend but you want her to be. just ask her is my advice. but the best time is when the date is just about to end (in my opinion).