Photographing a number of people together is as much about direction and stage management as it is about cameras and lenses. The principal challenge is getting everyone ready at the same time and co-ordinating all the positions, looks and expressions. People inevitably have their own views about the arrangement of the group, and someone will be distracted, make a joke or blink at the critical moment. Be sure to acquire additional safety shots. Call for attention with some authority only when everything is ready, but don't expect concentration to persist for more than a couple of minutes. If direction is not sufficiently robust the group may feel collectively in charge, and control will be lost. In general, try to steer a course between issuing military orders and being too jovial. Wedding photography is probably the ideal training ground.
A good starting point is to find a location that provides an attractive background and a focal point, such as a gate or fallen tree, around which the group can assemble. Then consider the group structure, bearing in mind the interactions and relationships of the individuals. Careful planning is required when three or more people are involved, because a row of soldiers lined up across the frame is just not normally interesting. Try varying the depth and height of the composition by arranging people according to size and seating children at the front. Build pyramids of people, for example where children flank a dominant adult figure, then merge several of these building blocks into a larger pyramid.
Lighting is less critical for a group than for individual portraits because each face is less significant in the image. Nevertheless, it is still a key consideration for the overall image. Use diffused light to reduce contrast, and beware of shadows cast by one group member onto another. Lens selection will depend upon the group's size and location, but a focal length of 50mm or 85mm makes a good start. Remember that depth of field extends approximately one third before and two thirds beyond the focused distance, so focus one third into the depth of the group.
If you have no control over the group an opportunistic approach is the only option. Ideally, position yourself somewhere between the group and the main event - the band, the food or whatever it may be. You will then get at least some attention. If the direction of the light makes this impractical, seek out a compromise position to one side.