I've created a business that tries to do this before. Well, the business was an events aggregator in NYC, but we envisioned that would be one of the future features.
When you learn how to force people to check-in to a club and broadcast it, you'll have this app. Until then, it's really difficult.
I once had an idea was that you would open up the app and it would have two buttons: (1) MALE (2) FEMALE. Clicking one would check you into that club. This would solve what a lot of people want -- where are the people & who are those people?
Even something that simple though -- what's the reward for the user checking in? On Foursquare, it was awards and badges and stuff. If you could do that around night-life, you might have something.
You have to make this sort of thing effortless (symbolic rewards lose their appeal over time). E.g, use background location or iBeacon so users are checked in without having to pull out their phone. The first time they enter a club, they get prompted for permission to check them in now and any future time they're at the club. Specifying male/female should only need to happen the first time they use the app.
I have on the my todo list a skip the line at clubs app of which I see there are many although I think I may have a novel idea for it. More something I want to build out of frustration at the horrible club line experience than because I think it's a good business model.
Check out BarEye in the App Store. I think they've narrowed down to a single geo now for advanced features like "send a drink", but the basic features like "which club are my friends at" work everywhere.
-_-