In a formation, it's generally accepted that the lead pilot is responsible for safety of flight and navigation. #2 is, colloquially, responsible for not hitting lead. That is to say, in close formation, you never take your eyes off lead. In many situations, #2 may not even be in radio communication with lead or monitoring ATC frequencies and his overall situational awareness will be degraded.
If lead, who is responsible for navigation, takes the flight into protected airspace (i.e, flies into Class C airspace without first establishing two-way radio communication) would both pilots be violated, or just lead?
The regulations governing formation flight are pretty sparse. FAR 91.111, 91.146 and 91.153 are the only mentions of formation flying in the regulations. There is no requirement for communication during the flight - 91.111 just requires that all pilots agree to operate in formation. It is not unusual in a formation flight for lead to be on one frequency (talking to approach), and the wing to be monitoring another (or not monitoring at all). Most communication is done via hand signals. I do know that ATC treats formation flights differently - they effectively treat it as one aircraft (ATC Manual 2−1−13). If the flight is cleared to land, both aircraft a cleared to land - #2 does not need to acknowledge. Hence the question who is responsible if the flight (a single aircraft in the eyes of ATC), violates an FAR?