The granulator is a sound generator, it has no effect on incoming signals. For that kind of thing, best look to FAUST.
To get the Scriptnode Granulator playing polyphonically, you have two options.
Increasing the density and reducing grain size will allow you to play chords, keep in mind though, anything after the granulator will still be monophonic.
For true polyphony, implement multiple granulators in different groups, one for each voice of polyphony. Then use a round robin script to handle the voice allocation.