Depends: if it's "the shop of the grocer" then it's "grocer's". But if there are two of them then it's "grocers' ". Also let's not forget the creeping use of "it's" when associating something with something else; so it's "The lion and its tail", not "The lion and it's tail"; the latter would mean, in full, "The lion and it is tail". Thus the title of this thread, in full, is "Concorde is in formation", which only makes sense if there was only one Concorde. So it's either, "A Concorde's in formation" or "Concordes in formation".
Grammar School education wasn't wasted after all.