Final answer:
The lead rider's hand signal with the left arm bent at the elbow and two fingers pointing up indicates a single-file formation for group riders.
Step-by-step explanation:
When a lead rider's left arm is bent at the elbow with their index and middle fingers pointing straight up, it signifies a single-file formation. This is a common hand signal used in group bicycle riding or motorcycle riding to communicate to the rest of the group to form a single line, often for safety reasons such as approaching traffic or road narrows. It is one of the standardized signals that help maintain order and safety within the group.