That is, the main variable that gets you to lower rates, is higher participation. Anything else doesn't come close.
You have to let it sink in that you can not have a small subset "save up" enough to pay for "their share." It just doesn't work without having large groups lose out and be completely unable to cover the costs in their group. Instead, this would be akin to group gambling where eventually one loser will bankrupt the entire pool.
Now, there is a related concept that you need to diversify your risk pool such that you don't fall prey to the same risk on everyone. This is what hit some major flood insurance providers after Katrina. I could see similar arguments for not wanting to insure a bunch of folks from the same locale. (Oddly, insurance doesn't typically branch out beyond locales...)
There is a concept of trying to get the higher risk pool to pay more through some rough proxies (typically age, for car insurance) to pay higher premiums. Again, though, most young drivers do not have accidents. Nor do the insurance companies typically actually need such high premiums to cover costs. As evidenced by their fairly mediocre savings plans they offer folks for not having an accident.
All of this, though, really just points out that calling this an "insurance" for health is just silly and is ultimately the wrong model.