Group Pivot Tables by Many-to-Many Linked Fields

I noticed that pivot tables cannot be grouped by a linked field if the linked field allows linking to multiple records.

You can see that the “Originators” option has an arrow, but no additional options appear under it:

My use case is to show a leaderboard of sales reps who funded the most this month. The component is set to the Advance object, which contains a field called “amount” ie. the funded amount.

I want to show the total funded this month, grouped by rep. But I can’t group by rep, because each Advance can be associated with multiple “Originators” and therefore it’s not groupable.

(My workaround was to create a rollup field on Users to sum up their associated Advance amounts, and then to use a regular table set to the Users object. But this only works for all time - there’s no way to filter a rollup field to just this month.)