The group this shape belongs to. A shape starts with no group and can be assigned a group once only; the group then cannot be changed for the lifetime of the shape. Grouping marks the shape's contact and trigger begin/end events with whether they are the first or last event between the two groups involved.
See PhysicsWorld.CreateGroup and PhysicsWorld.eventGroupingAllowed. Attempting to assign an empty group, or assign to a shape that already has a group, produces a warning and leaves the shape unchanged.