When you make your bid on Priceline you must agree (by making a specific indication) to their legal terms in line with the offer you make. They specify that you are bidding for a room at a rate for up to two people. Bringing in a third violates the terms of the bid.
This is absolute hogwash! See the terms posted above by another poster. The room is "guaranteed" for two but nothing says you are bidding at a "rate for up to two people."