You already have a good answer on separation specifically, but it's worth noting that there are a few general rules about formation flights in 14 CFR 91:
The most specific formation rules are in 91.111:
§91.111 Operating near other aircraft.
(a) No person may operate an aircraft so close to another aircraft as
to create a collision hazard.
(b) No person may operate an aircraft in formation flight except by
arrangement with the pilot in command of each aircraft in the
(c) No person may operate an aircraft, carrying passengers for hire,
in formation flight.
Charity flights are exempt from air carrier (part 119) and drug/alcohol testing (part 120) requirements, but only if the flight is not a formation flight (among other conditions):
§91.146 Passenger-carrying flights for the benefit of a charitable,
nonprofit, or community event.
(4) The flight is not an aerobatic or a formation flight;
You must provide formation details if you're filing a VFR flight plan:
§91.153 VFR flight plan: Information required.
(a) Information required. Unless otherwise authorized by ATC, each
person filing a VFR flight plan shall include in it the following
(2) The type of the aircraft or, in the case of a formation flight,
the type of each aircraft and the number of aircraft in the formation.
(3) The full name and address of the pilot in command or, in the case
of a formation flight, the formation commander.