I am writing this article for a friend in the US, who is planning a trip to Japan.
For a comprehensive list of the articles I have written for foreign people living or traveling in Japan, see this article.
Line (often stylized as “LINE”) is the de facto standard instant messaging service in Japan. Perhaps you could think of it as the Japanese version of WhatsApp? A recent study shows more than 80% of those in Japan who uses the Internet uses Line. They provide clients extensively for various platforms: not just mobile clients, but also desktop clients for Windows (both a traditional desktop version and a Windows Store version) and Mac OS, and even a Chrome browser extension (with limited capabilities). You can make and receive voice and video calls with it too.
Everybody uses it here in Japan and uses it for all the communication purposes. That means if you for any reason wanted to keep close contact with people in Japan, then you’d probably want to use it too. For individual communication, you can use any communication method you and your communication partner can agree on, say, Facebook Chat or Hangouts, but for group communication, Line is their go-to service, and if you do not use it, then it is very likely that you will be left out.
According to the Wikipedia entry on it, “the service is operated by Line Corporation, a subsidiary of Korean internet search engine company, Naver Corporation.” I have heard Line is also popular in South Korea too.
Your Line account is strongly tied to your phone number, so from the long-term perspective, probably you’d want to create an account with your US phone number in advance.
You do not necessarily have to use its mobile apps to sign up — you could do that with its desktop clients instead. However, you will still be asked for your phone number, and they will verify it by sending you a text with a code and asking you to type in the code on your desktop client.
You’d want to get connected with your friends in Japan soon so you will be able to start arranging plans with them beforehand, but that’s a bit trickier than you think. Your Line ID will not be searchable by them so you cannot just give your Line ID for people to contact you; you need to prove you are an adult for your Line ID to be searchable, but Line depends on age verification services provided by some Japanese cell phone carriers, which you have no access to in the US.
What you could do instead is to ask for their Line IDs so you can search and find them. Alternatively, you can invite them via email which will include a QR code. They can then scan the code to get connected with you, but they have to do this immediately, because the code is valid only for five minutes or even less.
The QR code actually decodes to a URL in the form of
https://line.me/ti/p/XXXXXXXXXX, so if you give me the QR code, I can decode it for you and you can just pass around that URL in the textual format, which may make things a whole lot easier for them. Otherwise, some people will have difficulty scanning the QR code shown on their phone screen with the very same phone (They could use QR Code Reader: PDF, Screen, Images Scan & Create on Android (free as far as this feature is concerned) or Scan on iOS (~$2 USD), both of which is capable of decoding QR code images in your camera roll.)