make sense to say that there any term that denotes 
an absolutely indivisible group of individuals ?

They are individuals, right? so you can individuate them, right?
so you can tell them apart. So if there a a group of them, that
means you have decided not to tell them apart. You can give a name
to any group, even if it is longwinded like

TheGroupOfIndividualDustParticlesThatLandedOnMyHeadOn13-Dec-2001

but now that I have decided to name that Group, it is indivisible as
far as that name is concerned. If 