Informal:
* Kudasai - This is the most common and polite way to say "come in" in informal situations.
* Hai - This literally means "yes" but is often used as an informal way to say "come in" when someone is knocking on the door.
Formal:
* Douzo - This is a more formal and polite way to say "come in".
* Oide kudasai - This is the most formal and polite way to say "come in", often used in business settings.
Other options:
* Irasshai - This is a common greeting in shops and restaurants meaning "Welcome" but can also be used as a way to say "Come in".
* Hajimemashite - This means "Nice to meet you" and is often used as a way to say "Come in" when someone is entering your home for the first time.
The best way to say "come in" will depend on the context and your relationship with the person you are talking to.