When you and your friends are going to a convention, a Halloween party, or otherwise need a costume, there's a great deal of satisfaction in going together, rather than individual, unrelated characters. Acting in-character is often encouraged when dressed up, and when you have a group of friends to act with, staying in character becomes both easier and far more entertaining. Harry Potter group costumes are easy and recognizable, but no less fun.
Harry Potter Costumes
Harry Potter is popular enough that those who are more unfamiliar with it will still recognize your group. They are fairly simple: Hogwarts students and teachers need robes. Robes with the Hogwarts emblem screen-printed on the chest will add accuracy. Wands ought not be forgotten. Your Harry Potter will of course need glasses and a little make-up for his scar.
However, your group may consider other aspects for an extra layer of detail. Consider wearing house scarves, or bringing a broomstick. Your Ron may consider bringing a rat with him, or your Harry a snowy white owl. Perhaps one of your Hogwarts characters will have the Maurader's Map or your Neville will bear Gryffindor's Sword. Just the right accessory can add an entire new element to your costume.
Consider also what characters you want together in the group. Perhaps you will want a dementor or a Death Eater, even Voldemort mixed in. Group dynamics change how each individual will act and how your group is perceived as a whole. Creative arrangements and props will catch the eye of others attending the event and together, your Harry Potter group costumes will be the best.