Have you ever been in really crowded shooting conditions? Common examples may be a meet and greet, or conditions in which the public may be wandering about with little regard for what you may be trying to accomplish. Believe it or not, you can get reasonably good results with a compact beauty disk, a light weight light stand, grid, and of course your camera and speedlight and remote trigger.
A 22" beauty disk can be purchased for around $50. Grids unfortunately can cost as much as the beauty dish itself. A 20 degree or less grid is typically ideal for this situation. Once you mount your dish/grid on your lightstand and attach your speedlight, you are ready to go. In a crowded room, it may be difficult to even get a few feet of wall space in a corner but with setup you have this limitation beat. Some speedlights communicate directly with certain cameras; if not you will need a remote trigger on your cameras hotshoe.
Place the beauty disk close to your subject (2 feet) and adjust your camera to a shallow depth of field. Adjust your camera for a shallow depth of field (F 4 or less) and assure that your shutter speed is set to the synch speed of your flash (at least 1/125th of a second or higher)
The grid will radically limit the light spread, and given the fact that you have your beauty dish very close to your subject, the light fall off will be rapid. You will want to frame your subject reasonably close and your final image will be similar to that of an image taken against a dark background. Even people as little as 5 feet away from your subject should not appear within exposed within your image.
If you do not have a beauty dish/grid, you can experiment with other modifiers that significantly restrict the spread of light from your speedlight or strobe. Remember to keep your subject close to the light for rapid light fall off as well as keeping your camera set for a very shallow DOF. Try this technique the next time that your are pressed for space!