Someone mentioned above, but if you have ad/abductor machines available they are pretty gd!
Stuff like jump squats, Olympic lifts (e.g. clean and jerk, snatch etc.) and deads all create strong powerful legs.
Really the best way to strengthen them for ground fighting is literally freeplaying (rolling at 100%) with your opponent in your guard and they have to escape/go for a sub. Your goal is to sub from your back, sweep or just control their posture. If guard is broken just repeat! Pretty intense on the adductors/general body !
2012 FFL Team : Hector Lombard, Jake Shields, Demian Maia, Ed Herman, George Sotiropoulus