I've always taken this to mean their personal ladies maids who are acting on the Bingley sisters' orders. I can imagine these London upper servants being an awe-inspiring sight, and very much aware of their status in the household.

And, if they are visitors, who could they be? Jane was the only lady in town that they could admit to being slightly worth their time. I can't see anyone else having the nerve to visit Netherfield uninvited. And then to gain admittance to the room of a sick guest without being escorted there by the hosts? If only so the Bingley sisters could swan in with a "look who has come to visit you" announcement.