I have a Module with List of Persons who made contributions as a row dimension along with their respective Contribution in a separate Line Item, now I want to calculate how much percentage share of each person in my second line item.
Does the list of people who contributed have a top level? If not, add one. The percentage line item formula would be: Contribution (line item) / Contribution[Select: Employee.All Employees] where All Employees is the top member.
That's the best and the easiest way of rolling up numbers as @rob_marshall suggested. There is another way but that will break most of the elements of PLANS where you will be using LOOKUPs. So let's not go there.
To answer your question - SUM can't be used in such scenarios