OK, but how are you wanting to rank the users? Are you just summing the total time for all sub-tasks of all tasks and ranking based on that? Are you summing the time for the sub-tasks of each task, ranking the users per task, summing the ranks for all tasks, and then ranking the users based on that? You need to explain how you want to get from a series of tasks, sub-tasks and times to a rank for the user. :)
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer