A seated guest and a person standing, turning and walking to the door are different events to a dog. Do not wait to see whether the next nip breaks skin. A guest should be able to leave without passing a dog who may rush their heels.
Before departure, place your dog behind a secure gate or in a comfortable room with the door closed. Give the guest a clear path. Do not ask them to toss treats, test a cue or keep moving past a barking dog. If someone has been bitten, seek appropriate medical care and get qualified, reward-based behavior help for an individual safety plan. A new pattern, especially after a move or other change, also merits a veterinary discussion.
Once departures are safe, note exactly when the change starts: the guest leaning forward, a chair scraping, shoes going on or the door opening. A professional can help practise those smaller parts without involving a visitor in a risky encounter. The visitor and doorway guide can support a broader arrival and departure routine, but it does not replace the barrier while nipping remains possible.
For the next visit, set up the dog's safe space before anyone arrives and tell the guest how they will leave. The first success is an uneventful exit.
Sources: Dogs Trust on aggression toward visitors and safety. Dogs Trust on preparing dogs for visitors.