actually increases their odds of getting asked on a date to the dance. They guy knows he won't have to be in a 1 on 1 situation all night long if he pairs up with one of the girls. He may technically be the "date" of one of the girls, but he'll also be just part of the larger group, which puts less pressure on them.
The girls in my area start planning their homecoming and prom groups a couple of months in advance. Girls who have steady boyfriends basically tell the boyfriends who the group is, and they're expected to be part of it. As it gets closer to the dance, a lot of the girls get formally or informally paired up or asked out, but it's not unusual to see a group photo of 7 girls and 4 guys or 8 guys and 3 girls. It's a good solution to bridge the gap that definitely exists today for teens in the dating world.