SET @running_total:=0;
select 
    xx.*,
    (@running_total := @running_total + xx.total) AS running_total
from (
select count(id) total, concat(concat(year(created_at), '-', lpad(month(created_at), 2, '0'))) year_month
from users 
where created_at is not null group by year_month order by year_month
) as xx;

From http://stackoverflow.com/questions/664700/calculate-a-running-total-in-mysql