Just to clarify, unless you are right beside your Trainers phone you will not be able to synch to the FitBit.  Instead what you need to do is add the app to your phone and synch to it.  You will be the only person who can see your info.  My son has borrowed my fitbit a few times and it works just fine.  Btw, I'm 52 yrs old and not overly tech savvy......but this one is really a no brainer.