OK, travelling as a group that large completely changes things! I've never travelled as a group that big and to be honest I doubt I even know that many people total!

I honestly don't think travelling as such a large group is a good idea. Most guides recommend groups of less than 6, which even splitting up you will be well over. The reason for this is you will run into issues with everything. For example if you want to take the train you will practically fill up an entire train car. Most restaurants will not be able to seat you, and those that can will require a reservation. The best restaurants are very often the small hole in the wall ones that seat 10 people and half are taken by regulars!

As far as accommodation goes you probably want to stick to hotels for the simple reason that you would have to book multiple Airbnbs anyway so at least with hotels you can have all the rooms together rather than everyone spread out all over the city.

I'm sorry if I'm sounding negative here but honestly travel in such a group sounds like a nightmare!

I believe the SUICA card is for JR rail where is the MetroCard is for the Metro around Tokyo. Anyway you can buy it at the ATM machines when you get here no rush

Line is a messaging application that's very popular in Japan. So when you meet people you will want to be able to talk to them via line.

You might want to load up the hello talk application. And meet some people in the places you're going to visit have some conversations with them and then when you're in town you'll have a local that you can bounce ideas off of and perhaps making sure you around a little bit.