How to ask a girl to dance?

I went to my first dance (10th grade year) and I danced with a few friends that were girls, and I wanted to know how can I ask a girl to dance when shes dancing already with a Friend, (kind of like interrupting) Just what should I say to her when it's a slow dance kind of thing. I want to know this for my next dance.How to ask a girl to dance?
Just say, ';Excuse me. May I cut in?'; Or ';May I have this dance?'; Make sure you are asking the both of them so that you can get the approval from both people. If you get a yes, you can cut in that way. Good luck!!How to ask a girl to dance?
just ask polite not loud and rude just be like hello miss can i have this next dance with u.
if you're slow dancing, ask politely. also a tip: if you're grinding, whatever you do, don't come up behind the girl and grab her really ';low,'; if you know what i mean. i had a guy do that to me at homecoming and i was really uncomfortable!!