The other people you see working out in classes with you, are called Ghosts.

Whenever you take a class in FitXR, your performance is recorded to create a "Ghost". Your Ghost is used to take part in the classes of other Users taking the same workout you've already done.

Whenever you take a class with Community mode on, the other players you see are all real people who've previously taken the same class - you're seeing the Ghosts of their workouts.