Do not hold the camera in the horizontal orientation on individual people, or groups of two people, unless the composition is actually, really,truly wider than it is tall! The vast majority of single-person close-up, head and shoulder, and half-body shots will look best with the camera oriented in the vertical (tall) orientation.
If there is an actual, solid, obvious reason to photograph a single person with the camera held in the horizontal orientation, then go ahead and do it. A farmer standing in front of a field of his prize-winning tomatoes behind him, with the field occupying some of the area--well, that would make sense.
But a shot of a small child, plopped in the middle of a horizontal frame, with two areas of empty space on either side of her head is an ugly, amateurish composition about 99.5% of the time.
Once your groups start to become larger than two, then you have to decide how to pose them so that the group's arrangement matches the camera's frame area; a group of 10 people will usually be easier to arrange in a wide-format group than a tall and narrow group arrangement.