Question: what is the difference between get in and come in?
Answer: Both 'Get in' and 'Come in' can be said as an invitation by a person sitting inside a room, a car etc. But if a person who is outside the room or car wishes to tell someone to enter the room or car, they can say 'Get in', 'Step in', 'Go in' or anything equivalent, but not 'Come in'. That's the difference.