Converting to date and performing a count
Don't know about big query but for SQL Server you could do it like this.

select S.UserID,
       min(cast(S.SessionLogin as date)) as SignUpDate,
       datediff(day, min(cast(S.SessionLogin as date)), getdate()) as DaysActive,
       count(distinct cast(S.SessionLogin as date)) as DaysActive
from dbo.SessionLogin as S
group by S.UserID;