D.Walker and E.Vul from the University Of California had this to say:"the fact that the visual system represents objects as an ensemble, individual objects are biased toward the ensemble average, and average faces are perceived to be more attractive than faces in isolation. Together, these phenomena should cause faces in a group to appear more like the group average than when presented alone, and that group average should tend to be more attractive than the individual faces, on average."

In other words, people are regarded as more attractive when they're in a group because you don't focus on imperfections as much and the brain averages out beauty. This is important in magazines because models must look visually appealing if you want to maximize sales. This applies especially to women who have almost impossible beauty standards constantly pushed on them by magazines.