One thing that comes to my mind is "Please, come in" or just "Please" (this is what I use mostly). Come in suggests me that you are standing in a room, but what if you are in a corridor?
What do you say in these cases? Formal or informal. Would be great if you could bring a few examples of what you say to your friends jokingly, like is it possible to say something like "fly in" or something along these lines? :) Thank you.