Iterating through generic collections

Question

I have a method that uses two generic lists. The first is a 'catch-all' list that holds id's for all projects, and inside the loop a test is run that adds the project weight to the other list. If this was all, everything would be ok, but it's not. I have to iterate through this for each user id and add their project weights to the same index of the list. So, instead of using the Add() method, I need to add amounts to the list's elements by index. However, I keep getting an ArgumentOutOfRangeException. The index is 0, and I'm not understanding where this is coming from.