In reply to teapot:
The taco is a good point. We had the same problem when we hired a mini bus to drive to Font for a club trip. Because of the taco hours we needed two drivers.
To the OP. We've tried all sorts getting to and from Font from trains (including shared taxis from Font Station to Millett - walk from there!), taking cycles on the train,mini bus hire and private cars, and the latter has always come out tops in terms of convenience and price. Although it's nice to travel as a group it takes a lot of organising and isn't necessarily the cheapest or most convenient option. Our club now tends to block book the camp site and and let individual members make their own travel arrangements, mostly sharing with car owners. I've never looked into the possibility of hiring a car at Font station, but might be worth considering, although Paris would have more choice.
I've never tried the Boulder Bus. Does it still run? If so, maybe they might do a special run for a club?
Post edited at 09:58