in the illustris-3 at z = 0 , we have many members with "GroupNsubs" = 0 (and = 1). Is it true? and what dose it mean? ( is it means the group has no member??)
GroupNsubs is a property of (FoF halo) groups. It gives the number of (Subfind subhalo) subgroups in that halo.
If GroupNsubs==0 then the FoF halo has no Subfind subhalos. This is common for the lowest mass halos in the catalog (with ~32 particles in the FoF halo), because the Subfind algorithm is unlikely to find a bound substructure in that halo with more than the minimum number of required particles (20).
If GroupNsubs==1 then the FoF halo has just a single Subfind subhalo. We call this the "central", and this just means that there are zero "satellites", which is also normal.
Just another note, typically we want to impose a minimum resolution requirement for halos, depending on science application. This could be 50, 100, or 1000 DM particles, or star particles (just as examples). This is because FoF halos with only 32 particles are at the resolution limit and so are not trustworthy.
If you make such a restriction you will typically not see GroupNsubs==0 cases.
Thank you Dylan