Having a little trouble with a query provided by Periscope. Can you help point me in the right direction?

Error is - ERROR: aggregate functions are not allowed in GROUP BY Position: 305

with monthly_activity as (
  select distinct
    date_trunc('month', created_at) as month,
    user_id
  from oauth_refresh_tokens
), 
first_activity as (
  select user_id, date(min(created_at)) as month
  from oauth_refresh_tokens
  group by 2
)
select
  this_month.month,
  count(distinct user_id)
from monthly_activity this_month
left join monthly_activity last_month
  on this_month.user_id = last_month.user_id
  and this_month.month = last_month.month + interval '1 month'
join first_activity
  on this_month.user_id = first_activity.user_id
  and first_activity.month != this_month.month
where last_month.user_id is null
group by 1