Formation Flights
Do not fly close enough to another aircraft to create a collision hazard under any circumstances. Outside of this restriction, formation flight are allowed by prior arrangement between the pilots involved in the formation.
Dropping Objects
Another very straightforward regulation. No pilot may allow anything to be dropped from the airplane that could create any kind of hazard to people or property.
§ 91.111 Operating near other aircraft.
§ 91.15 Dropping objects.