fireflybob,
Be very careful with your wording and the use of "passengers". You cannot take up a passenger unless the establishment holds an AoC (Air Operators Certificate). If one is in place then this can be deemed a passenger carrying service.
If it is a training flight, the pliot in command/instructor can allow "observers" to sit in the back. This subject to the usual weight and balance restrictions and at the discretion of the pilot in command. They are there merely to observe the person in the front enjoying their training flight (and hopefully get something out of it as well).
I would question any flight school who try to put a surcharge on taking observers along. It just takes the p**s a bit.