Board Thread:Questions and Answers/@comment-174.0.49.47-20140930025818/@comment-25470191-20140930193108

Not many know for sure - C.C and Jeremiah definitely knew,Sayoko, Lloyd and Cecile most likely knew and were a part of the plan, beyond that I don't think he needed to inform anyone else to execute the plan as Geass was more reliable than trust.

After it happened, Nunally knew due to super-empath-psychic power, Kallen figured it all out. I expect that, due to intelligence, Xingke, Schneizel and Kanon would've worked it out as well. It's quite likely that all those who knew Lelouch was Zero realised the switch and the purpose of it but this is all speculation.