Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Strength
MGGROUPEFFECTOR_STRENGTH
Parameter: Selection
MGGROUPEFFECTOR_SELECTION
Parameter: ReTime
MGGROUPEFFECTOR_RETIME
Parameter: Start
MGGROUPEFFECTOR_STARTTIME
Parameter: End
MGGROUPEFFECTOR_ENDTIME
Parameter: Effectors
MGGROUPEFFECTOR_EFFECTORLIST