I think the club would still be stuck with some liability.

It's like an auto dealer who sells you a car. They are always held to a higher standard than an individual who sells you a car. If the car had a safety problem, and you crashed, the law (or the jury) is more likely to find that the dealer "should have known", and disclosed or fixxed the problem.

The same kind of standard would likely hold against the club, vs. a few friends getting together to ride. I agree with you, that a grown person should be able to sign away this liability. But, I wouldn't want to be caught running a club without good insurance.

I can't see much liability from posting "Anyone want to go for a ride?", unless there is some illegal aspect, or bad intent. I would take the chance, although I suppose you still could be sued in some circumstances, under some tortured "should have known" logic.