Why does a 24 year old girl hang out with a 40 year old couple?

My girlfriend and I know this 24 year old girl for 6 years and she hangs out with us and goes to movies or dinners with us. I like her alot and want to ask her for only us to go out but can't seem to do it. Her and my chick text from time to time but I want to text her. I'm not happy with being with my chick and want to be with this other girl. I just wonder why she would hang out with us. When she had a boyfriend we didn't here much from here now she's single but has somewhat of a busy life but she will go out with us. Last time we went out to a movie she said she's excited. She hugs my chick but seems as she wants to hug me or waiting on me or something. I want her to know I like her but don't want to freak her out. Wonder if she will hang out with me alone? Just wonder why she does. Last time we had a friend like this she would invite me over to her appt. But I don't know about this one. She quite. She kept her movie ticket last time. Any clue ideas ECT. What can I say or do?