Are you sure you know what you are doing? (Applying a Layer Mask to a Group is not the same as applying the same Layer Mask to all contained Layers.)
Does this not work?
var id495 = charIDToTypeID( "Dlt " );
var desc95 = new ActionDescriptor();
var id496 = charIDToTypeID( "null" );
var ref66 = new ActionReference();
var id497 = charIDToTypeID( "Chnl" );
var id498 = charIDToTypeID( "Chnl" );
var id499 = charIDToTypeID( "Msk " );
ref66.putEnumerated( id497, id498, id499 );
desc95.putReference( id496, ref66 );
executeAction( id495, desc95, DialogModes.ALL );