Hi there,
Unfortunately, the highway buses can take only 27 passengers at once. Also, as you mentioned, the night bus from Shinjuku to Ogizawa is running only on weekdays.
Alternatives are you can take a night bus from Shinjuku to Toyama station and start Alpine Route from Toyama side to Ogisawa, instead of from Ogizawa to Toyama.
You can take a train from Toyama station to Tateyama and start Alpine Route.
There are 2 night buses running from Shinjuku to Toyama on weekends.
1. Depart Shinjuku at 22:25
2. Depart Shinjuku at 22:50.
As 30 people cannot take the same bus, so, half of your group member can take the 22:25 bus and the rest can take the 22:50 bus.
However, the reservation site is Japanese only, so you might find it difficult to book your tickets.
If possible, it might be easier to call the bus company and make a reservation.
The phone number is +81 76-433-4890
Reservation website is:
https://secure.j-bus.co.jp/hon/RouteList/List?uncd=1801&name=%E5%AF%8C%E5%B1%B1%E5%9C%B0%E6%96%B9%E9%89%84%E9%81%93
I hope you enjoy your trip to Alpine Route.