Not counting C.C. and Lelouch in any of these for obvious reasons.
>People who know the Geass.
Kallen, Suzaku, Wakamoto, Orange-kun, Viletta, Rolo.
>People who know Zero's identity
Kallen, Suzaku, Wakamoto, Orange-kun, Viletta, Rolo, a few disposable OotBK members that IIRC died.
>People who know Lulu is a prince
Suzaku, Wakamoto, possibly Rolo, probably Orange-kun, Millay (though possibly not anymore since the memory-editing), obviously the whole royal family. Kallen ought to know as well since Nunally became Governor General, but she never seemed to catch on.
There may be a few I'm forgetting.