Hi! Looks like you have some of ASL's structure already down 👏
You could sign: many people here wow.. you want leave? You, me dinner, us finish? (For a yes/no question: eyebrows raised, leaning forward slightly). This loosely translates to: There's too many people here, you wanna go and get dinner? Just the two of us?
Best of luck asking her out!
For signs: http://www.lifeprint.com/
https://www.handspeak.com/