Bidders & Passwords
If you are doing online bidding then by default bidders will be prompted to create a password when they sign up.
This is control by a setting on the registration element.
Auctria associates a password directly with an email address, not a specific event. If a bidder registered in a previous year, or for a different event and set up a password at that point, then they would be required to enter their password again to register rather than setting up a new one.
It is not possible to change the bidder's password as an event organizer, since the password may be associated with multiple events the bidder is involved in. Any page that asks for a password will also offer a Forgot your password link though.
See Bidders in Online Auctions for more details on other ways bidders can be identified in online event.