please find attached a possible solution.
On the mastersheet, ALL agencies are listed. It should be possible to select various agencies for teh different investors which should be then put on the respective investor sheets (sheets named "Investor 1","Investor 2", "Investor 3" in my file).

If u check these sheets out, you fet a feeling how they should look like. Important: it should be possible to select a agency more then once. Example: As in my file, Agencie "AAAA" shows up both, on "Investor 1s" sheet as well as "Investor 2"s sheet.

Hope thats more clear...

test_sheet_empty_adjusted.xlsx