Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Strength
MGGROUPEFFECTOR_STRENGTHParameter: Selection
MGGROUPEFFECTOR_SELECTIONParameter: ReTime
MGGROUPEFFECTOR_RETIMEParameter: Start
MGGROUPEFFECTOR_STARTTIMEParameter: End
MGGROUPEFFECTOR_ENDTIMEParameter: Effectors
MGGROUPEFFECTOR_EFFECTORLIST