There was an interesting thread about Empty Club Syndrome on the official forum, under Showtime. I can't link to it because it refers to a mod, so here's a brief recap of suggestions.
1. If you have a restricted club, remove the red ropes and the bouncer.
2. If your club doesn't have people living nearby, you'll be less likely to get people in the club.
3. People won't start showing up until after you arrive, so wait about 30 Sim minutes.
4. Elevators can slow things down, so add another elevator or put in stairs.
How many people you can get will be determined by your computer's CPU rating, which is found under documents, in the device config file.
If your CPU is 1, the maximum of Sims on the lot is 4; if it's 2, the maximum is 9-10. I don't remember 3 and 4, but you get the idea.
