filter on invoices by usergroup

In our company we are using a division where each user belongs to a main usergroup.

When we select “invoices” “list” instead of filtering by “sales representative” and “member connected” we need to filter by usergroup ( the usergroup that the user logged in belongs ).
I have dived in the database structure and migrated from mysql to postgres , but I don’t know anything about PHP and where exactly i could modify the source pages.
Any indication would be welcome.

More over I would need to have the selected usergroup the logedin user belongs preselected as the filter to apply to the invoices.
Suggestions ?