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.