When a bidder is out bid they will receive a notification on their mobile device:
Tapping on the notification will open the item in the Auctria app so they can see the current bid and place a new bid.
At any point the bidder can also use the Messages page from the menu to review the notifications they have been sent:
You can also send bidders notifications from the website via Communication → Send Notification. These will pop up on the bidder's devices and appear in their messages tab