<p>May I answer a question you didn't ask? That question is how to go about shooting a commercially acceptable group shot of 30-35 people.<br>
Rather than worrying about buying a lens, I'd urge you to think through the shot in other ways. First, you really want to shoot outdoors, because I'm assuming you don't have professional flash units, light stands, umbrellas, wireless triggers, etc., etc. A good shot needs good light and creating it takes practice and expensive equipment. Just cranking up the exposure index isn't a good substitute. If speedlight flash is all you've got, you're going to have trouble getting even exposure of the group from front to rear. If bouncing off the ceiling is possible, that would help, but it will cut two stops of light and require a larger aperture, hence shallower depth of field.<br>
Never set up a location shoot without scouting the location, ideally before you quote a price. That's asking for trouble until you see what will be involved. You want an attractive or at least neutral background in the shade. Is there a better location close by? Maybe it's slightly less convenient for people to walk a bit to the location, but they're paying you to get a memorable photo not to make things as convenient as possible. You're the director of this shot and they are the actors. If you have confidence and are politely professional, they'll go along with your ideas. If it's feasible, try to insist that the shot be scheduled in the morning or afternoon, when you can pose the group in shade. Nice even open skylight will prevent blocked shadows and blown out highlights and you may be able to add daylight-balanced fill flash to add catchlights to eyes and open up any shadows. Outdoor lighting will also allow you to use a faster shutter speed to prevent camera shake and a smaller aperture for greater depth of field.<br>
<br />A tripod may or may not work for this shot. You'll want to be taller than anyone and able to look down on the group, after posing taller people in back and shorter in front. If you can't get above the group using what's already there, be prepared to bring a step ladder or at least a tall step stool to raise your point of view.<br>
Pose with the final print in mind. If you're going to sell 4x6 or 6x9s, your viewinder will be fairly accurate, if 8x10's you'll need to compose accordingly.<br>
Now to the lens. What's wrong with the 18-55? Your knowledge is more important than the lens you use. My long term suggestion would be to join the Professional Photographers of America (PPA) and attend seminars geared toward portrait photographers. The cost of doing so will improve your business-oriented photography faster than a new lens.</p>