Bidders must have an account in order to place online bids.  

If you have sent Invitations to Bids from the software, the link provided will automatically provide the user with an account and log them in so they arrive at your site ready to bid.

Users that didn't receive an invitation can use the (optional) Signup buttons provided to create their own account.  Signup buttons can be enabled (and customized) at Admin > Site Settings > Online/Mobile Bidding > Signup and Invitations.