We noticed a flash of lightning that just occurred at the north of our track. Using the WX radar we were only able to paint light to medium precipitation at north of our track. Looking ahead visually we saw nothing but clear sky and stars above, but there was no moon yet and it was very dark. The dispatcher said in an ASARS (aircraft communication addressing and reporting system) message just minutes before that all the rides ahead had been reported smooth. All of a sudden the overspeed EICAS and aural warnings went off and the captain immediately retarded the throttles and turned off the auto throttles and autopilot. He told me to turn on the seatbelt sign and the ignition switches to continuous. He was battling very hard as we immediately started climbing at about 3000 fpm. I then told the flight attendants over the PA to take their seats immediately. The captain finally got control of the aircraft as we were approaching FL370, and then eased it back down to FL350. The turbulence we encountered was moderate to severe as for a few moments we were unable to control the aircraft. During the steep climb I made a transmission to any other aircraft that might be along our track, but fortunately we were ahead of the track that night. We talked to the flight attendants right away and it has aggravated a previous back injury. No passenger was even out of their seats so that was good. No other flights behind us encountered anything other than a little light to moderate chop. We notified commercial radio immediately and had them pass everything along to dispatch, who talked with us eventually on a phone patch. We made a logbook entry and after a cabin inspection and group discussion we deemed the aircraft able to continue on to our destination.4.Who told the flight attendants over the PA to take their seats immediately?