There are two things which should be taken into account when selecting a lens for group portraits or any other scene where minimal distortion is desired:

1) Distance to the subject affect focal length: The ideal lens is the one which provides the desired in-camera crop at the distance where the facial proportions look most natural: 8ft or further

2) DSLR lenses are astigmatic: Wide angle lens tend to distort and stretch the edges of a photo. That is actually a design feature to correct the curvature of field. The plane of prime focus for a simple lens is actually an arc. The optics of the DSLR alter the optical path so everything will be sharp on the flat plane of the sensor, but in the process the edges get stretched. By way of analogy think of a tennis ball cut in half and nail to a board with a nail in the center. Now stretch out and nail the cut edges to a rectangle on the board. The shape of the ball near the center will be about the same but the ball will be distorted more towards the edges.

How do the two relate when shooting groups? With a wide group the facial perspective may look OK at 8ft, but the zone of astigmatic correction / distortion on the edges will create unacceptable distortion. As you move further back you will be able to use longer focal lengths to get the same in camera crop and those longer focal lengths will have less edge distortion. But you could also just move back with the same lens that caused distortion at 8ft. From a greater distance the group will occupy only the center portion of the frame which has less distortion.

But please note that in either case the solution requires increasing the camera-subject distance. Indoor your studio space will be the limiting factor. So experiment with the lenses you have at various focal lengths and distances and find the one which works best in the space you have.