My first golden was like that--wouldn't really play that much with other dogs, even as he got older. He was more of a "people dog." When my breed club would host events, he'd be out swimming with the other dogs, but always off to himself. He'd get excited about running after a ball with a pack of dogs, but wouldn't play keep away, or take away--he'd bark to let you know he wanted the ball thrown again--even if he wasn't the one who got the ball.
My second golden was a "social butterfly." She had to greet all people and dogs and other animals. Everyone had to be her friend. She was extremely laid back, and would just take in the action, greet one and all, and have a great time mingling.
Appreciate your pup for the golden that they are becoming; they leave us far too soon. Both of my goldens were seniors, and they passed away within 2 years of each other; my current pup, now 16 months old, is like a pleasant blend of both of them. As others have said, as long as your pup is not stressed or afraid, enjoy these times together. Sounds like you have a very smart golden to be used as the class demo dog already!