How could I find the void, filament and group regions in the illustris box?
Does il.groupcat.loadHalos(basePath,snapnum,'GroupPos') get the group positions?
There aren't any results from a "cosmic structure finder" available, e.g. that would identify voids, filaments, and nodes, either by where they are spatially in the box or by labeling which galaxies reside in each region.
There are many codes and algorithms which could be applied to this task - they could operate on either the particle-level data, or as you say, directly at the halo-level data.
Yes if you load all GroupPos or SubhaloPos this will give you the positions of all the objects, from which the structure of these different web components could be derived.