Dan brings up a really essential part of preparation for anyone leading a group: screening the trip members. The wrong person on a trip could ruin the others' enjoyment, truncate the route (if they're out of shape), or even get himself/herself or others into dangerous situations. The Sierra Club Outings people do lots of training on member screening, but even so sometimes people will slip through.
It's less of an issue if you're not a group leader, but it's still important to know who you're hiking with, and what their capabilities and limitations are. (Also helps to have a sense of personality and hiking style.)
I've made a few mistakes in screening, and at least one difficult decision that turned out the be absolutely right.
On my first trip there was a 72-year-old guy who looked great on paper, but if I had looked more closely I'd have realized his recent trips were all much less strenuous than ours; we hiked him out after the first day, so it didn't have much of an impact on the trip.
On another trip there was a woman who had suffered a head injury a couple of years before but said she was fully recovered. By the second day she was having serious trouble with her balance...and this was on trail, and we had days of off-trail travel to come. Before the trip, she had completely glossed over any lingering effects of her injury (which, besides balance problems, included occasional paranoia and other delusional thoughts). We got her out, and continued with the rest of the group.
I had one trip with two guys (buddies) who were just complete jerks: constantly slamming me behind my back, second-guessing every decision I made, and generally being relentlessly negative. Part of the problem, I later realized, was that the older of the two was not in nearly as good shape as he thought he was...but instead of admitting it to himself he made his problem my fault. The two guys wrote in formal complaints (the only ones I ever got), the Club surveyed the other trip members, and the upshot was all but one other trip member complained about them.
The one time I really dodged a bullet, it was a tough call. She was a doctor, and I always liked to have a doctor on the trip--it just gave me an extra level of confidence in the event of injury or other medical issues. She also had a chronic condition that didn't adversely impact her ability to do the hiking (she was in great shape, very active, and did a lot of really strenuous travel), but did impose some very complicated dietary limitations. She wanted to come along but bring her own food to cook herself; that, of course, isn't how it works on Sierra Club trips. I probably could have worked it out with her, but over the course of the conversation I got a really bad feeling about her personality, and a strong sense that if she came along she would be real trouble. I turned her down, and the food thing was the pretext, but the real reason was this sense I had that I couldn't put in concrete terms.
That year or the next, she did go on someone else's trip...and was a complete nightmare. She single-handedly ruined the trip for a lot of people and was, in fact, the reason that leader stopped leading. So in that case, at least, my gut feeling was absolutely right.