How to group records in mysql query
This query should do it for you

select 
    tm,(tm + interval 15 minute) tm2,user,users
from 
(
    select tm,user,COUNT(1) users from
    (
        select user,(access_time - interval
        MOD(unix_timestamp(access_time),900) second) tm
        from user_access where access_time between
        '2013-05-28 02:00:00' and '2013-05-28 10:00:00'
    ) AA
    group by tm,user
) A;


I'll leave the formatting to you

Give it a Try !!!