Nemusíte „vytvářet tabulku s hodnotami za 12 měsíců“. Můžete to udělat v dotazu:
SELECT m.mon, COUNT(i.date_added)
FROM (select 1 as mon union all select 2 union all select 3 union all select 4 union all
select 5 union all select 6 union all select 7 union all select 8 union all
select 9 union all select 10 union all select 11 union all select 12
) m left outer join
invite i
on m.mon = i.month(date_added) and year(date_added) = 2013
GROUP BY m.mon;