Simple steps:
1. Get her number
2. Wait a few days
3. Ask her out
And that's it. It's really that simple. Shall I elaborate?
1. Get the number - if you ask her for the number and she gives it to you willingly, it means she's interested on some level. It could be friendship level or considering you as a dating partner level, but either way there's interest there. Asking for the number is also the most cautious way of gauging her interest without having to spill the beans about your level of like for her.
2. Wait a few days - Don't do with her number like the other 99 guys did that she's now not dating. Get the number, give it a few days (I wait 4, others will wait shorter or longer than this, but definitely not the next day), then hit her up.
3. Ask her out - I tend to do it via phone call. Some guys on here will say you should text her, while I feel that texting a girl to ask her out seems kinda wimpy. Regardless of how you contact her, don't spend a lot of time on the phone with her - just call her and ask her to do something with you. Have a place and two times picked out from her to choose from (i.e. "I was hoping I could take you out to this Indian food place I found, are you free on Tuesday or Thursday?") Then shut up and wait to hear her response. Either she'll jump at it ("oh, that sounds great, Tuesday works perfect") and you'll know she's interested; or she'll make up an excuse with no counter offer ("oh, sorry, I'm busy both those days") and you'll know she's not interested.
Regardless of how it goes, when you ask her out,
do NOT tell her how much you like her. Just asking her out will let her know you like her on some level; the part that makes them feel more attraction to you is trying to figure out HOW much you like her, and what she needs to do to make your like for her go up even more. Plus, if you really think about it, you know her based on seeing her in a group setting and haven't really had any one-on-one time with her, so your like is based purely on speculation. Dial it down a bit and wait until you actually take her out BEFORE making or feeling all these love proclamations to her. Hope this helps!