Trek to mount fuji

As of now i'm planning to take the tokaido shinkansen line to shin fuji station. from there i'm going to have to take a bus to fujinomiya 5th station. i'll be reaching shin fuji station at about 6 pm on 27th August 2014. Would i have to do the bus bookings in advance as we're 20 people travelling together? if yes, can you suggest how to do the bookings?
Also, we are planning to rent a hut at the 5th station so that we can dump our excess luggage there and proceed towards the trek at around 10 pm. Again, is advanced booking necessary, or will we get a hut there easily? In any case can you provide me the details regarding the booking of the hut and bus.
Thank You.

As of now i'm planning to take the tokaido shinkansen line to shin fuji station. from there i'm going to have to take a bus to fujinomiya 5th station. i'll be reaching shin fuji station at about 6 pm on 27th August 2014. Would i have to do the bus bookings in advance as we're 20 people travelling together? if yes, can you suggest how to do the bookings?

The last bus from Shin-Fuji Station departs at 17:45, so you'll either need to arrive earlier or book alternate transportation.

Also, we are planning to rent a hut at the 5th station so that we can dump our excess luggage there and proceed towards the trek at around 10 pm. Again, is advanced booking necessary, or will we get a hut there easily?

Advance booking is necessary for 20 people. At this late stage you may not be able to get a reservation.

In any case can you provide me the details regarding the booking of the hut and bus.

They accept reservations by phone, and likely in Japanese only. It's the last listing on this page:

http://www.fuji-tozan.com/04_lodge.html

Actually, I'd strongly consider either chartering a bus and driver, or send your luggage ahead to your next destination. Then you can skip the problems booking a hut.

What is the rest of your itinerary?

Is there a way in which we can do the bus bookings in advance? There's a chance that there might be less than 15 people.

You need to make a plan for climbing by April. Then make a hut reservation in May/June, make bus reservation in July/August. This is because Mt Fuji is not a hiking course, is a climbing course.

You can use a taxi from Shin-Fuji Station to Mt Fuji 5th station which cost you between 10,000-11,500 Yen by making advanced booking(normal taxi fees will be 12000-14,300 Yen), 4 passenger can be on board.

Non hut reservation climbing on Mt Fuji is high risky especially a group of 15-20. I don't recommend your climbing plan. You also need to send a climbing report to Shizuoka Police Department in case of death and injury while you are climbing.

Wouldn't the bus booking problem be solved if we buy the Fuji trip and back pass offered by JR? Because it includes access to trains and buses upto the 5th station. I found this info here:
http://www.jreast.co.jp/e/fuji/