group  ; date       ;  quarterTime    ; persons_in ; hours
group1 ; 17-02-2010 ; 00:00 - 00:15  ; 0          ; 0
group1 ; 17-02-2010 ; 00:15 - 00:30  ; 0          ; 0
group1 ; 17-02-2010 ; 00:30 - 00:45  ; 0          ; 0
~etc
group1 ; 17-02-2010 ; 13:00 - 12:15  ; 334        ; 1483

end then the same for all the groups starting with 00:00 until 23:45

SELECT
        CONVERT(bigint,Year(GETDATE())) * power(10,7) +
        Month(GETDATE()) * POWER(10,5) +
        Day(GETDATE()) * POWER(10,3) +
        Datepart(hh,GETDATE()) * POWER(10,1) +
        DATEPART(mi,GETDATE()) / 15 AS Agg_Date