Bill senders and receivers at Customers → Referenced Customers.
Customers → Referenced Customers (/customers/referenced) lists bill senders and receivers used on cash bookings.
Who can open it: users granted Referenced Customers. There is no page-level Add — parties are created from the cash order form (and similar booking flows).
Open Manage. The workspace matches cash customers (Details, Orders, Invoice, Settings), with fields preset for that sub-role.
You expected Add on this page: create the party while booking a cash order instead.