These are non-negotiables. You can't have a small group without them. How these elements are put together, however, vary. For a kite to fly high and far, it must be tethered. Without being kept firmly in-hand or held to the ground, a kite will become swept up in gusts of wind but soon spiral down and drive itself into the ground.
Small groups are like kites.
They aim to soar high but they need to be tethered to a contract or agreement that clearly articulates the purposes for the group. Many groups want to fly free and easy, betting that serendipity and spontaneity will provide enough to keep on track. These groups usually don't last very long. They have a huge initial burst of speed, but they soon lose momentum. The best groups are not put together haphazardly but assembled in a deliberate, well-thought out fashion.