Unfortunately, most nightclubs require that you order at least one bottle in order to reserve the table, and only then can you order a la carte cocktails / shots / beers after that bottle. The reason is because ordering a la carte drinks places undue pressure on your table server and the bar staff during the prime hours when the club is slammed.
That being said, some dayclubs (especially in Vegas) do allow a la carte ordering to hit your minimum spend (dayclubs have more extensive food menus and more bartending staff on duty so they are better equipped to take individual drink orders from table parties).