He'll be attending for a voluntary interview (VA), it'll be recorded either on tape or video. My guess is that he's a potential witness, or if they have any evidence from other sources that he may have been driving dangerously, he may be a suspect. The reason for a VA is that after the event, the necessity to be arrested may have passed.
Other persons can be charged if there's enough evidence. This is a quick google, I know it's car drivers, but the situation could be seen as similar.
http://www.nottinghampost.com/two-teenag...story.html