I've had one guy who kept getting groin strains while doing kick boxing. First I had him rest until the problem had time to heal, then I began to add leg adduction & abduction moves . You may have a machine in your gym, or you can use a pulley for adduction work http://www.exrx.net/WeightExercises/Hip ... ction.html
(if that's not an option you could do "Bench scissors")
You can do abduction work by laying on your side attaching weights to your ankle & lifting the leg.
I started him stupidly light, working at 20 rep range, then gradually worked up. You don't need to go super heavy & he hasn't had another strain since.