About the same as Gsquared - either should work, I think

select cityname as City, groupname as [Groupname],

isnull(subqty.qty, 0) as Qty

from (select groupid, group.name groupname, city.name Cityname, cityid

from dbo.Group CROSS JOIN city) cartesianX

left outer join

(select cityid, groupid, count(*) as Qty

from dbo.User

where joindate >= @startdate

and joindate < @enddate

group by cityid, groupid) SubQty

on cartesianX.cityid = subqty.cityid

and cartesianX.groupid = subqty.groupid

order by cityname, groupname