We've been using OpenERP v7 for some time, but also recently added an Odoo v9 installation and I can confirm that this feature is present in the new POS module -- simply tap the logged-in user's name at the top of the POS screen to switch the salesperson that the current sale is assigned to. In our instance, we open and close the session with a generic store user, but the floor staff switch to their name whenever they put a sale through. This allows us to track the effectiveness of each salesperson in retail clothing store.

Technically this should also be possible by scanning a barcode (follow the barcode nomenclature then add the barcode as the EAN in the salesperson's user card) but I could never get it to work under v7 and I have not yet tried it under v9.