Just go up and ask them.
And it helps if you already are their friends if you want to ask them out.
Just get to be her friend then just ask her out.
Simple as that.
Obviously she is already interested in conversing with you so use that to your advantage.
That is the best way to go and most people will appreciate if you have the courage.
Just try to find her when you two are just hanging out by yourselves and ask!
Tip :Never Hesitate