Summing Column values based on group by criteria
So to get the hours from the two timestamps as they were summed in the group by (note that tt_timeadjust is by default, in minutes) I used 

SELECT SQL_CALC_FOUND_ROWS *, ROUND((SUM( TIMESTAMPDIFF(MINUTE, tbltimetracker.tt_begintime, tbltimetracker.tt_endtime ) + tbltimetracker.tt_timeadjust)) / 60, 2) AS hours FROM tbltimetracker LEFT JOIN tblcustomers on tbltimetracker.customer_id = tblcustomers.customer_id LEFT JOIN tblusers on tbltimetracker.user_id = tblusers.user_id WHERE  tbltimetracker.owner_id = ?  GROUP BY tblcustomers.customer_id tblusers.user_id LIMIT 0, 50