Few crowd simulation algorithms take collective group features such as group formations into consideration. Recent approaches often manually specify hard constraints such as predefined keyframe agent formations and interpolate them during the simulation. A proposed interactive, scalable framework generates freestyle group formations and transitions via natural and flexible sketching interaction. It automatically computes a plausible agent distribution in the target formation and agent correspondences between keyframes. In addition, two-level formation trajectory control lets users intuitively guide agents' transition paths from the initial formation to the target formation. In experiments, the framework efficiently generated formations on the fly while providing sufficiently flexible control to users.