No, just because the attendee has a credit card on file and a bidder ID does NOT mean that they have a portal account created, which is required for online or mobile auction bidding. 

However, if your Event Settings page is linked to the auction and you have enabled Send Portal Account Info Email, then the ticket purchaser and each attendee with an email address that was provided at the time of purchase will receive an email link with their portal account information.

Since ticket purchasers often do not know the names of their guests (actual attendees), those guests will not receive the automatic email notification if their guest name and information is updated on the Event Attendance grid after the purchase is complete.

Also, tickets are frequently purchased several weeks prior to an event, and the attendees may not log in to the portal at the time they receive the email. They may also forget they received it or it could go into their spam folder. You can validate if a portal account already exists for a guest by looking in the Portal Users grid.

Portal users can update their password themselves by clicking on Forgot Password or you can reset it for them by clicking on Reset Password from the Portal Users Grid.

From the auction page URL, those with a portal account will be able to choose the sign in option, those without, will need to select sign up.

For those with a portal account, the sign in looks like below. Note that attendees with a bidder number, their last name and a credit card on file can log in with that information as well.

For those without a portal account or without a bidder ID and credit card on file, they will provide their name, email, and create a password and click sign up. They will be taken to a new screen to enter their credit card information.