Hi - I faced this same problem only yesterday. It's a bit long winded, but should sort the issue
1) You will have to split up the "grouping" list into smaller lists of 9999 each. You can do this by creating a module dimensioned by the "grouping" list.
Add the following line items:
Count: formula = 1
Rank: formula = rank(count, ascending, sequential)
List 1: Formatted as your grouping list, formula = if rank<10000 then name(item("grouping" list)) else blank
List 2: Formatted as your grouping list, formula = if rank>10000 and rank <=19999 then name(item("grouping" list)) else blank
etc....
You can then create separate lists based on line item List1, List2 etc. via an import.
2) in your ranking module create a line item for the original grouping list and a number of line items formatted again the newly created lists;these will be for the ranking groups. Set the formulas of the to use a "finditem" against newly created list. The groping list should only appear in one of the line item.
3) Add a number of rank formulae using the ranking groups from 2) and then sum them together to give you the overall rank.
