Passports and Japan!!?

Travel Info
Ok, I have a Japanese passport and my friends are coming to Japan with me, but do they have to have a japanese passport but have American passports. Do they have to have jaspanese passports as well? or can they just use there american passports. We are probably going to go on American AirLines and Japanese AirLines. Please answer my question and tell me other information if you know more!

Travel Tips

鈾t doesn't matter if you have a Japanese passport and they have US passports. You may all fly together, sit together and arrive in Japan together. The only place you go your separate ways for a 'short time' is when you pass through Immigration. You will go through the Japanese nationals lines (people who have Japanese passports) and they will go through the Foreign passport holders line. They both exit at the same place, so just wait for each other then proceed to pick up your luggage together and go through customs one by one. You'll have no problem, don't worry. I fly with my Japanese friends who have Japanese passports sometimes. I have a US passport.
As already mentioned, Americans may stay in Japan for up to 90 days.
Hope this helps! Have a safe and wonderful trip. Have fun!鈾?
Others

You carry the passport of your nation of birth.
Yes of course they can use their American passport, and it would be very hard for them to get a Japanese one anyway. If you mean a visa, then I think it isn't needed (for the UK this is the case, so probably the same for the US) check on your local embassy's website.
The carry the passport of the country that they are citizens. They need to check if they need a visa or not to go with you to Japan.
You can all go together. No problems. Americans in Japan can stay up to 90 days without a visa.
Mia, you need to learn a little more about the world and international travel. I have never heard such an airheaded question!!!