Results for every hour of the day even data if not present
Take the query you already have and capture the output in a temp table #T.

select cast(T.creationDate as date) as ForDate,
       datepart(hour, T.creationDate) as OnHour,
       count(distinct T.userId) as Totals
into #T
from dbo.T
group by cast(T.creationDate as date),
         datepart(hour, T.creationDate)


Get the distinct ForDate values from #T and cross apply against a list of hours for one day and left outer join #T to get the Totals for each hour.

select dateadd(hour, H.OnHour, cast(D.ForDate as datetime)) as ForDate,
       isnull(T.Totals, 0) as Totals
from (select distinct T.ForDate from #T as T) as D
cross apply (values(0),(1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),
                   (13),(14),(15),(16),(17),(18),(19),(20),(21),(22),(23)) as H(OnHour)
  left outer join #T as T
    on H.OnHour = T.OnHour and
       D.ForDate = T.ForDate

drop table #T