Group foto gotta watch out for DOF, if too shallow, some people's face will be OOF. So ask the following questions to yourself:
How many people? Sitting/standing in row? Or can you estimate the distance between the persons nearest/farthest from the cam? How far are they from the cam, as in focus distant (this may vary if you take full or half body). With these info, find out the DOF you need, with a fixed focus distant and focal length, you can find out the aperture you need.