Sit in the butterfly position commonly used as a stretch. Using either your hands or arms to provide resistance, or holding a dumbbell or kettlebell on each knee, lift your knees up and then lower under control as far as possible. If rehabbing an injured groin, slow eccentrics are a good idea.
 
Purpose
Butterfly adduction is a simple and easily controlled adductor strength, rehab or prehab exercise.
 
Programming
Use 2-3 sets of 10-15 reps.