And what happens if one of your 5 friends is diagnosed with an incurable but expensively treatable illness? Do you kick them out? Do you increase the premiums? Do you invite more people in to the pool?
Nowadays, health insurance is really "assurance" sprinkled in with a doctor subscription (as daniel-cussen mentioned). With that in mind, a hypothetical plan design would collect premiums that bundle reserves, a concierge medicine program, like HealthTap, and critical illness insurance, cancer, and/or accident insurance. These insurances only cover specific incidents for the immediate term. Once someone triggers one of these events, you could push them onto the exchange where the larger risk pool would then cover it on an ongoing basis.
Whether or not that would be ethical is up to you...
From an economical standpoint, depending on your area's availability (read urban centers), you could likely put together this entire package for a collective and pay the ACA tax for less than an ACA approved plan.
You might have included a clause where you can release a member from the pool for a high one-time cost from each of the other members.
At the end of the day, you would undoubtedly be liable to pay in one form or another for a portion of those treatments, unless the friend felt bad and voluntarily left the insurance group.
Insurance among friends would be weird, mixing emotional and purely mathematical calculations.
When participating in a larger insurance group, these same costs are born solely by insurance premiums. But the costs are not the same - big insurance companies have negotiated lower prices across the board. Your group cannot.
I see p2p insurance as a total loss, because in time of need they cannot possibly pay.
The whole point is that everybody has coverage under a high deductible standard insurance plan (which will be significantly cheaper than a low deductible plan), and then the group shares the risk of the deductibles.
Best case, everybody pays less for insurance (including what they pay into the p2p pool). Worst case, someone gets sick and busts the pool (and gets kicked out if it's chronic), but everybody still probably paid less overall. Not sure how well this works if the person who got booted can't get their deductible reduced.
I think health insurance is weird in a number of ways for this. It'd probably be great for e.g. homeowners insurance.