At some point, you're going to have to pull her away from her friends for a little one on one. If you've followed our advice, you should be able to slink away from the group and get some alone time in with no protests from her friends.
Rather than making promises about an elaborate date you want to take her on, or laying on some cornball shit about how you really want to "get to know her mind," be honest with her. We suggest saying, "I'd really like to see you again." There's something refreshing about how straightforward that is.
At which point, she will hopefully offer you her phone number. What you do with it then is up to you, but if she gave it to you, I can tell you what she wants you to do with it—actually call her.