the general concept of a formation flight plan is where the aircraft maintain a CLOSE proximity flying as if they were a single aircraft for the flight. You put the details of the other aircraft in the remarks field.
However I would suggest that if you are flying under conditions where you need a flight plan than filing one per aircraft is a much simpler method.
I guess you are looking for a way of filing a flight plan when you are leading a bunch of aircraft on your trips? If that is the case then they are not a formation.