Aggregate Function On SubQuery

I am working on a view in SQL Server 2005.
I am trying to get a list of the number of sessions each user had by user. I tried doing it this way, but


SELECT userid, MAX
((SELECT COUNT(DISTINCT sessionId) AS SESSIONCOUNT
FROM dbo.Sessions AS OD
HAVING (sessionId = O.sessionId))) AS MAXSESSION
FROM dbo.Sessions AS O
GROUP BY userid

but it throws an error 'Cannot perform an aggregate function on an expression containing an aggregate or subquery.'

Is there an elegant solution for this?

Thanks,