Reporting Services Grouping

Answer 1

Just use:

=Sum(Fields!time_difference.Value)

and it will sum over the group correctly (in this case, just for the employee).

Adding the data set's name takes the sum of the value over the entire data set as you have discovered. This is useful for calculating percentages, for example:

=Sum(Fields!time_diff.Value) / Sum(Fields!time_diff.Value, "GetEmployeesOverTime")

This would calculate the sum for this employee (that is, this group in your table) as a ratio of the entire firm (that is, the entire data set).