With the module activated, if you open Party -> Parties from the main menu, select a party, then switch to Form view, you should find the Sale Invoice Grouping Method field under the Sale tab.
Beside adding a new grouping method, you have to add also some logic to do the actual grouping. So first look a the current code in the sale-invoice-grouping. Nan-Tic has also some modules NaN-tic · GitHub which adds some extra grouping methods. No idea if those will work, but you can look at the code and see how it works.