"Will you take our picture?" the hopeful people ask you while they are on vacation. By shooting with the proper framing, you can really make their day! (Of course, they probably won't even realize it, but you can rest well assured of a job well-done.)
Generally speaking, for shots against backdrops, you'll want the subject's heads in a line one third of the distance from the top of the frame. If the heads are centered, there is often too much sky in the shot.
If there is something of note in the background that the people are standing in front of, consider giving it more space in the frame, too. For instance, put the people on the right side of the frame, and the Leaning Tower of Pisa on the left.
So you have the subject's head at the top third of the photograph... now where do you cut them off? At the torso? Waist? Feet? It depends on what you want. If you want to show their face, then torso-up is good, because their face will dominate the photo. If you want to show them in relation to their surroundings, you might zoom back. For instance, if your subject is standing next to an elephant, you might want to zoom back all the way so their entire body is in shot, just for a size comparison!
If the subject is smartly dressed, and you want to draw attention to what they are wearing, consider turning the camera vertically so that you can photograph more of their outfit and keep it aligned with the frame.
Another general guideline for framing: leave empty space (photographers call this "negative space") on the part of the frame your subject is facing. This is more of a guideline than a rule—pushing your subject up against the edge of the frame can have its own positive effect in certain cases. Be artistic!
If you don't get it right in the shot, don't worry! You can always crop the photograph later in Photoshop.