People are drawn together by a shared sense of identity. That identity can be based in almost anything—a science-fiction TV show, a nationality, a shared crises, a hobby, a love of Starbucks, or a desire to save the whales.
For my small group, it was tea. Being greeted with a kettle of hot water and several flavors from which to choose was welcoming and soothing to our somewhat harried souls. Because it was a cheap and easy way to communicate care, we latched onto it and grew it into an identity. We gave tea as gifts, brought flavored sugars and honey with us from home, and kept fun mugs on hand for each other to use. Even those who weren't big tea drinkers grew to love it for its sense of identity.
Another group discovered that they shared a passion for motorcycles. They rode them to their meetings and to other things they did as a group. Those who didn't start out with a bike soon got bit by the bug, and the group turned their passion into a mission, ministering to motorcyclists who didn't know Christ.
Sometimes it's as simple as a shared love of chocolate, an interest in gardening, or a passion for photography. Sometimes group members know they share this interest from the start and design a group around it. But often, these interests develop more subtly and need to be nurtured into an identity. Become the small group that swaps fiction books, works with computers, or eats jelly beans. It doesn't matter what it is—intentional investment in a common interest over time can go a long way toward creating a group identity.