Let's say a girl wants to dance with you. Usually you have to approach her, take her hand, or whatever, but if you go over just near enough and perhaps make some eye contact, she will feel relieved and close the rest of the distance -- again, if she already wants to dance with you. A fair amount of the time, she will actually approach you. It's always preferable for her to do at least some of the work because, before she can begin pursuing you even partly, she must shut off the chilling worry-spigot and allow a blood-warming bath of impulses to wash over her. That's how she enjoys herself in a social gathering -- by getting lost and swept away in the moment.
Teenagers may be more boy-crazy than late 20-somethings, but they are also much more self-conscious in public, so you might think that a 16 to 18 year-old girl would never be able to overcome her anxiety and approach you. However, girls, especially young ones, don't even go to a public bathroom as individuals. As long as she doesn't have to break away from her group, she won't get a panic attack. Thus, her entire group will come over and surround you.
The girl who wants you is always the one in the circle who's right in front of you, in case you couldn't figure that out (guys, eh?). Again, the others are there mostly to keep their friend from feeling overwhelmed by nervousness, although any of her friends who also like you will make this known. A few weeks ago, I was surrounded by six girls, and the one right in front of me had grinded on me earlier that night and returned with her comforting group. Still, two of the remaining five also wanted in, so I ended up with one girl grinding her ass on mine, and one girl grinding on each of my thighs. I only wish I had another pair of legs to offer the three remaining girls in the circle.
There is no greater way of broadcasting your value to other females in the venue than being pursued and engulfed in a pile of girls. For this reason, when this happens, it will usually happen more than once that night, as onlookers follow the original group -- "Look at that! That guy must be worth approaching." However, since teenagers are more ruthless in judging mates, you have to stand out right away. In a dance club, that means being painfully good-looking and demonstrating your prowess in coordinating your body. In a daytime situation, that means dressing and carrying yourself powerfully and confidently (although being ridiculously hot helps here too).
In the club for fully grown adults that I went to, girls had no problem leaving their group to approach me. By their mid or late 20s, they've had enough experience interacting with guys, and their level of Neuroticism has fallen from its teenage peak, so they aren't paralyzed by anxiety anymore. I would therefore expect the "surrounded by the group" phenomenon to not happen, or at least to happen much more rarely. I was only there one night, but I didn't observe it anywhere in the club the entire night. Broadcasting your value to the onlookers must be much more difficult in that sort of venue -- yet another reason to favor girls young enough for the herd mentality to still rule.
Perhaps the greatest aspect of this dynamic is that any potential cockblockers in her group will feel uncomfortable piping up. After all, she begged her friends, "C'mon you guys, go over there with me so I don't feel weird," and if they oblige, then they are prevented from interfering. It's all based on who approaches who: guy approaches girl, and her friends man their battle stations, while if girl approaches guy, her friends are conscripted to fight for the cause. If you approach her, you can try your damnedest to defuse her cockblocking friends -- it's the best you can do, but it rarely works.
This is one reason why I don't like the orthodox Game method of approaching a group of girls out of the blue and trying to win them over on the spot. Display some alluring quality of yours -- your looks, height, dancing ability, whatever -- and remain comfortably aloof, while still engaging in whatever the context requires (dancing, eating your meal, etc.). This provides enough time for the spark of infatuation to light within her, and she will then pursue you at least partly. Since that's the most important part of getting her friends off your back, as well as muzzling her inner worry-wart, it's worth doing.