"Shadow" meaning "a person who accompanies someone in their daily activities at work in order to gain experience at or insight into a job."
The term " to shadow "(跟随/跟著) means 'follow (closely like a shadow)'.
"Today is your first day on the job, you don't have to do anything, just shadowing John and watch how he works."
For a noun that describes 'someone who does the shadowing'
I suggest '見習生'
"you would be John's shadow today"
見習 (v) to watch and learn
見習生(n) a person on job probation (watch and learn)
見習生 is a general term, It can be a student training in actual work place or a new employee starting a job on probation. Different companies have different training methods. Shadowing is one kind of 見習, which trainee can learn by observing, and not hamper the trainer's regular work routine.