Check into FB
When you check into your workout it should prompt you to add it to your FB. It can say "Mrs.Jones is doing her workout with" either trainerize or if the client uses a custom app it can have their logo..
Then the trainer can set up a rewards program for checkins either based on giving a session away, free week of online programming or even a client program to see who checks in the most in a month.. So many options here and it gives the trainer free marketing to THOUSANDS of people.