PostgreSQL query to count/group by day and display days with no data

You just need a left outer join instead of an inner join:

SELECT d.date, count(se.id)
  FROM 
    (
    SELECT to_char(date_trunc('day', (current_date - offs)), 'YYYY-MM-DD') AS date 
       FROM generate_series(0, 365, 1) AS offs
    ) d
  LEFT OUTER JOIN sharer_emailshare se 
    ON d.date = to_char(date_trunc('day', se.created), 'YYYY-MM-DD')
  GROUP BY d.date;