What is a door person?
The old name was Bouncer. These were the security guards of years gone by. The Term Door person is now someone licenced (in the UK particularly) to be a security person on the access door to clubs, pubs and social gatherings.
If someone is knocking at the door, you could: answer the door answer the door and choose your response to the person answer the door and tell the person you are not interested (ex. a door to door salesperson) answer the door and invite the person inside (ex. a family member) answer the door but tell the person you're busy or give an excuse ignore the knocking
The person who opens the door in traffic, The reason why is when anyone exits their vehicle they should always look and be sure it is clear and safe to do so. If a person does open their door in traffic and causes an accident regardless of whether the door is damaged or not that person is liable for damages, injuries to property and people as a result of negligence.
The implication of 'at the door' is that a person is waiting to be given access or at least to talk with someone who is at home. It would usually be a person who is 'at the door'. "The repair man is at the door", "John and Mary are at the door." 'By the door' could refer usually to any object that happens to be near the door, inside or outside. "Where is mom's umbrella?"…