public.json: Add member count as drop.members

From this morning on IRC (time in PST):

07:39 < shofetim> re member counts, Christina felt that it was
  important to tell the customers how big the drop was, that people's
  comfort levels with a drop would be different depending on whether
  it was a gathering of a few friends, or something that more
  resembled a block party.  World readable permissions.